diff --git a/pages/registrieren.vue b/pages/registrieren.vue
index f5fbaaf..c3c8ecc 100644
--- a/pages/registrieren.vue
+++ b/pages/registrieren.vue
@@ -20,6 +20,9 @@
             <div class="text-sm text-gray-700">
               <div class="font-medium">Registrierungsmethode</div>
               <div class="text-xs text-gray-600">Passkey = Anmeldung ohne Passwort (z.B. FaceID/TouchID/Windows Hello)</div>
+              <div v-if="!isPasskeySupported" class="text-xs text-red-700 mt-1">
+                Passkeys aktuell nicht verfügbar: {{ passkeySupportReason || 'Bitte HTTPS/aktuellen Browser verwenden.' }}
+              </div>
             </div>
             <label class="flex items-center gap-2 text-sm font-medium text-gray-800">
               <input v-model="usePasskey" type="checkbox" class="h-4 w-4" :disabled="isLoading || !isPasskeySupported">
@@ -194,7 +197,7 @@
 </template>
 
 <script setup>
-import { ref } from 'vue'
+import { onMounted, ref } from 'vue'
 import { AlertCircle, Check, Loader2, Info } from 'lucide-vue-next'
 
 const formData = ref({
@@ -210,9 +213,21 @@ const errorMessage = ref('')
 const successMessage = ref('')
 const usePasskey = ref(false)
 const isPasskeySupported = ref(false)
-if (process.client) {
-  isPasskeySupported.value = !!window.PublicKeyCredential
-}
+const passkeySupportReason = ref('')
+
+onMounted(() => {
+  try {
+    const hasPKC = typeof window !== 'undefined' && typeof window.PublicKeyCredential !== 'undefined'
+    const secure = typeof window !== 'undefined' && !!window.isSecureContext
+    isPasskeySupported.value = !!(hasPKC && secure)
+    if (!secure) passkeySupportReason.value = 'Kein Secure Context (HTTPS oder localhost erforderlich).'
+    else if (!hasPKC) passkeySupportReason.value = 'Browser unterstützt WebAuthn/Passkeys nicht.'
+    else passkeySupportReason.value = ''
+  } catch {
+    isPasskeySupported.value = false
+    passkeySupportReason.value = 'Passkey-Check fehlgeschlagen.'
+  }
+})
 
 const handleRegister = async () => {
   errorMessage.value = ''
@@ -271,7 +286,7 @@ const handleRegisterWithPasskey = async () => {
   successMessage.value = ''
 
   if (!isPasskeySupported.value) {
-    errorMessage.value = 'Passkeys sind in diesem Browser/unter dieser URL nicht verfügbar (HTTPS erforderlich).'
+    errorMessage.value = passkeySupportReason.value || 'Passkeys sind in diesem Browser/unter dieser URL nicht verfügbar (HTTPS erforderlich).'
     return
   }