diff --git a/backend/models/index.js b/backend/models/index.js
index 9111d33..ca3337b 100644
--- a/backend/models/index.js
+++ b/backend/models/index.js
@@ -163,7 +163,7 @@ Club.hasMany(UserClub, { foreignKey: 'clubId' });
 Group.belongsTo(DiaryDate, { foreignKey: 'diaryDateId', as: 'diaryDateGroup' });
 DiaryDate.hasMany(Group, { foreignKey: 'diaryDateId', as: 'groupsDiaryDate' });
 
-GroupActivity.belongsTo(DiaryDateActivity, { foreignKey: 'id', as: 'activityGroupActivity' });
+GroupActivity.belongsTo(DiaryDateActivity, { foreignKey: 'diaryDateActivity', as: 'activityGroupActivity' });
 DiaryDateActivity.hasMany(GroupActivity, { foreignKey: 'diaryDateActivity', as: 'groupActivities' });
 
 Group.hasOne(GroupActivity, { foreignKey: 'groupId', as: 'groupGroupActivity' });
diff --git a/backend/services/diaryDateActivityService.js b/backend/services/diaryDateActivityService.js
index f965c0f..6c2be9c 100644
--- a/backend/services/diaryDateActivityService.js
+++ b/backend/services/diaryDateActivityService.js
@@ -208,6 +208,17 @@ class DiaryDateActivityService {
             
             // Auch für GroupActivities
             if (activityData.groupActivities && activityData.groupActivities.length > 0) {
+                const seenGroupActivityIds = new Set();
+                activityData.groupActivities = activityData.groupActivities.filter(groupActivity => {
+                    if (!groupActivity || groupActivity.id === undefined || groupActivity.id === null) {
+                        return false;
+                    }
+                    if (seenGroupActivityIds.has(groupActivity.id)) {
+                        return false;
+                    }
+                    seenGroupActivityIds.add(groupActivity.id);
+                    return true;
+                });
                 for (const groupActivity of activityData.groupActivities) {
                     if (groupActivity.groupPredefinedActivity) {
                         // Hole die erste verfügbare Image-ID direkt aus der Datenbank
diff --git a/frontend/src/views/DiaryView.vue b/frontend/src/views/DiaryView.vue
index 44db960..21c6bda 100644
--- a/frontend/src/views/DiaryView.vue
+++ b/frontend/src/views/DiaryView.vue
@@ -173,7 +173,7 @@
                                                         @change="assignGroupToActivity(item.id, $event.target.value)"
                                                         style="font-size: 10px; width: 80px;">
                                                         <option value="">Gruppe...</option>
-                                                        <option v-for="group in groups" :key="group.id" :value="group.id">
+                                                        <option v-for="group in groups" :key="group.id" :value="String(group.id)">
                                                             {{ group.name }}
                                                         </option>
                                                     </select>
@@ -199,8 +199,8 @@
                                             <div style="display: flex; gap: 0.5rem; align-items: center; padding: 0.5rem; background: #f0f0f0; border-radius: 4px;">
                                                 <select v-model="newPlanItem.groupId" style="flex: 1;">
                                                     <option value="">Gruppe auswählen...</option>
-                                                    <option v-for="group in groups" :key="group.id" :value="group.id">{{
-                                                        group.name }}</option>
+                                                    <option v-for="group in groups" :key="group.id" :value="String(group.id)">
+                                                        {{ group.name }}</option>
                                                 </select>
                                                 <input type="text" v-model="newPlanItem.activity" placeholder="Aktivität" 
                                                     @input="onNewItemInputChange" style="flex: 1;" />
@@ -257,7 +257,7 @@
                                                             @change="assignGroupToGroupActivity(groupItem.id, $event.target.value)"
                                                             style="font-size: 10px; width: 80px;">
                                                             <option value="">Gruppe...</option>
-                                                            <option v-for="group in groups" :key="group.id" :value="group.id">
+                                                            <option v-for="group in groups" :key="group.id" :value="String(group.id)">
                                                                 {{ group.name }}
                                                             </option>
                                                         </select>
@@ -317,11 +317,9 @@
                                         <div style="display: flex; gap: 0.5rem; align-items: center;">
                                             <select v-model="newPlanItem.groupId" style="flex: 1;">
                                                 <option value="">Gruppe auswählen...</option>
-                                                <option v-for="group in groups" :key="group.id" :value="group.id">{{
-                                                    group.name }}</option>
+                                                <option v-for="group in groups" :key="group.id" :value="String(group.id)">
+                                                    {{ group.name }}</option>
                                             </select>
-                                            <input type="text" v-model="newPlanItem.activity" placeholder="Aktivität" 
-                                                @input="onNewItemInputChange" style="flex: 1;" />
                                             <div v-if="newItemShowDropdown && newItemSearchResults.length"
                                                 class="dropdown" style="max-height: 9.5em;">
                                                 <div v-for="s in newItemSearchResults" :key="s.id"
@@ -396,7 +394,7 @@
                                     @change="updateMemberGroup(member.id, $event.target.value)"
                                     style="margin-left: 10px; font-size: 8px; width: 5em;">
                                     <option value="">-</option>
-                                    <option v-for="group in groups" :key="group.id" :value="group.id">
+                                    <option v-for="group in groups" :key="group.id" :value="String(group.id)">
                                         {{ group.name }}
                                     </option>
                                 </select>
@@ -985,16 +983,12 @@ export default {
             // Map für memberId -> participantId speichern
             this.participantMapByMemberId = response.data.reduce((map, p) => { map[p.memberId] = p.id; return map; }, {});
             // Map für memberId -> groupId speichern und mit Reaktivität initialisieren
-            this.memberGroupsMap = {};
+            const newMemberGroups = {};
             response.data.forEach(p => {
-                if (p.groupId) {
-                    if (this.$set) {
-                        this.$set(this.memberGroupsMap, p.memberId, p.groupId);
-                    } else {
-                        this.memberGroupsMap[p.memberId] = p.groupId;
-                    }
-                }
+                const groupValue = (p.groupId !== null && p.groupId !== undefined) ? String(p.groupId) : '';
+                newMemberGroups[p.memberId] = groupValue;
             });
+            this.memberGroupsMap = newMemberGroups;
         },
 
         async loadActivities(dateId) {
@@ -1327,7 +1321,7 @@ export default {
                         clubId: this.currentClub,
                         diaryDateId: this.date.id,
                         activity: this.newPlanItem.activity,
-                        groupId: this.newPlanItem.groupId,
+                        groupId: Number(this.newPlanItem.groupId),
                         timeblockId: this.selectedTimeblockId,
                     });
                 }
@@ -2117,9 +2111,10 @@ export default {
             if (!groupId) return; // Leere Auswahl ignorieren
             
             try {
+                const normalizedGroupId = String(groupId);
                 // Alle Mitglieder der gewählten Gruppe zur Aktivität hinzufügen
                 const groupMembers = this.presentMembers.filter(member => 
-                    this.getMemberGroup(member.id) === groupId
+                    this.getMemberGroup(member.id) === normalizedGroupId
                 );
                 
                 for (const member of groupMembers) {
@@ -2214,9 +2209,10 @@ export default {
             if (!groupId) return; // Leere Auswahl ignorieren
             
             try {
+                const normalizedGroupId = String(groupId);
                 // Alle Mitglieder der gewählten Gruppe zur Gruppen-Aktivität hinzufügen
                 const groupMembers = this.presentMembers.filter(member => 
-                    this.getMemberGroup(member.id) === groupId
+                    this.getMemberGroup(member.id) === normalizedGroupId
                 );
                 
                 for (const member of groupMembers) {
@@ -2232,28 +2228,29 @@ export default {
 
         // Gruppenzuordnung für Teilnehmer
         getMemberGroup(memberId) {
-            return this.memberGroupsMap[memberId] || '';
+            const value = this.memberGroupsMap ? this.memberGroupsMap[memberId] : undefined;
+            return value !== undefined && value !== null ? String(value) : '';
         },
 
         async updateMemberGroup(memberId, groupId) {
             try {
-                const selectedGroupId = groupId || '';
-                
-                // Verwende Vue.set für Reaktivität (Vue 2)
+                const selectedGroupId = groupId ? String(groupId) : '';
+
                 if (this.$set) {
                     this.$set(this.memberGroupsMap, memberId, selectedGroupId);
                 } else {
-                    // Vue 3 oder Fallback
                     this.memberGroupsMap = {
                         ...this.memberGroupsMap,
                         [memberId]: selectedGroupId
                     };
                 }
-                
-                // API-Call zum Speichern der Teilnehmer-Gruppenzuordnung
-                await apiClient.put(`/participants/${this.date.id}/${memberId}/group`, { 
-                    groupId: selectedGroupId || null 
+
+                await apiClient.put(`/participants/${this.date.id}/${memberId}/group`, {
+                    groupId: selectedGroupId ? Number(selectedGroupId) : null
                 });
+
+                // Synchronisiere die Teilnehmerliste, damit Anzeige und Datenlage übereinstimmen
+                await this.loadParticipants(this.date.id);
             } catch (error) {
                 console.error('Fehler beim Aktualisieren der Teilnehmer-Gruppenzuordnung:', error);
                 this.showInfo('Fehler', 'Fehler beim Aktualisieren der Teilnehmer-Gruppenzuordnung', '', 'error');