diff --git a/client/src/components/InvitationAcceptance.tsx b/client/src/components/InvitationAcceptance.tsx
index e28a3f5..e53d882 100644
--- a/client/src/components/InvitationAcceptance.tsx
+++ b/client/src/components/InvitationAcceptance.tsx
@@ -137,6 +137,7 @@ export default function InvitationAcceptance({ onAccepted, onCancel }: Invitatio
     const masterKey = getActiveMasterKey()
     const activeUserId = localStorage.getItem('active_userid')
     if (!masterKey || !activeUserId) {
+      autoAcceptStarted.current = false
       setError(isDe
         ? 'Sitzung unvollständig — bitte erneut anmelden (Benutzer-ID fehlt).'
         : 'Incomplete session — please log in again (user ID missing).')
@@ -184,7 +185,8 @@ export default function InvitationAcceptance({ onAccepted, onCancel }: Invitatio
           id: logbookId,
           encryptedTitle: rawEncryptedTitle,
           updatedAt: new Date().toISOString(),
-          isSynced: 1
+          isSynced: 1,
+          isShared: 1
         })
       }
 
@@ -202,7 +204,10 @@ export default function InvitationAcceptance({ onAccepted, onCancel }: Invitatio
   useEffect(() => {
     if (loading || accepting || autoAcceptStarted.current) return
     if (!isLoggedIn || !logbookId || !logbookKey || !token) return
-    if (!sessionReady()) return
+    if (!sessionReady()) {
+      autoAcceptStarted.current = false
+      return
+    }
 
     autoAcceptStarted.current = true
     void handleAccept()
@@ -240,10 +245,17 @@ export default function InvitationAcceptance({ onAccepted, onCancel }: Invitatio
     e.preventDefault()
     if (!recoveryInput.trim() || !encryptedPayloads) return
 
+    const resolvedUser = (username.trim() || encryptedPayloads.username || '').trim()
+    if (!resolvedUser) {
+      setAuthError(isDe
+        ? 'Benutzername konnte nicht ermittelt werden — bitte erneut anmelden.'
+        : 'Could not determine username — please try logging in again.')
+      return
+    }
+
     setLoading(true)
     setAuthError(null)
     try {
-      const resolvedUser = username.trim() || encryptedPayloads.username
       const success = await completeLoginWithRecovery(resolvedUser, recoveryInput.trim(), encryptedPayloads)
       if (success) {
         setShowRecoveryFallback(false)
diff --git a/client/src/services/db.ts b/client/src/services/db.ts
index 212a4c5..81379a5 100644
--- a/client/src/services/db.ts
+++ b/client/src/services/db.ts
@@ -5,6 +5,7 @@ export interface LocalLogbook {
   encryptedTitle: string
   updatedAt: string
   isSynced: number // 1 = yes, 0 = pending local modifications
+  isShared?: number // 1 = collaborator copy, 0 or unset = owned
 }
 
 export interface LocalYacht {
@@ -120,6 +121,17 @@ class DaagboxDatabase extends Dexie {
       gpsTracks: 'entryId, logbookId, updatedAt',
       logbookKeys: 'logbookId'
     })
+    this.version(4).stores({
+      logbooks: 'id, encryptedTitle, updatedAt, isSynced, isShared',
+      yachts: 'logbookId, updatedAt',
+      crews: 'payloadId, logbookId, updatedAt',
+      deviations: 'logbookId, updatedAt',
+      entries: 'payloadId, logbookId, updatedAt',
+      syncQueue: '++id, action, type, payloadId, logbookId',
+      photos: 'payloadId, entryId, logbookId, updatedAt',
+      gpsTracks: 'entryId, logbookId, updatedAt',
+      logbookKeys: 'logbookId'
+    })
   }
 }
 
diff --git a/client/src/services/logbook.ts b/client/src/services/logbook.ts
index f40fa7a..85dc456 100644
--- a/client/src/services/logbook.ts
+++ b/client/src/services/logbook.ts
@@ -43,8 +43,6 @@ export async function fetchLogbooks(): Promise<DecryptedLogbook[]> {
     throw new Error('Master key not found. User must log in.')
   }
 
-  const sharedLogbookIds = new Set<string>()
-
   if (navigator.onLine) {
     try {
       const response = await fetch(API_BASE, {
@@ -61,7 +59,6 @@ export async function fetchLogbooks(): Promise<DecryptedLogbook[]> {
         // Decrypt and save logbook keys locally if they exist
         for (const lb of serverLogbooks) {
           const isShared = lb.userId !== userId
-          if (isShared) sharedLogbookIds.add(lb.id)
 
           const encryptedKeyStr = isShared
             ? lb.collaborators?.[0]?.encryptedLogbookKey
@@ -105,7 +102,8 @@ export async function fetchLogbooks(): Promise<DecryptedLogbook[]> {
           id: lb.id,
           encryptedTitle: lb.encryptedTitle,
           updatedAt: lb.updatedAt || new Date().toISOString(),
-          isSynced: 1
+          isSynced: 1,
+          isShared: lb.userId !== userId ? 1 : 0
         }))
 
         // Clear existing cache for this user and insert new ones
@@ -128,7 +126,7 @@ export async function fetchLogbooks(): Promise<DecryptedLogbook[]> {
       title,
       updatedAt: lb.updatedAt,
       isSynced: lb.isSynced === 1,
-      isShared: sharedLogbookIds.has(lb.id)
+      isShared: lb.isShared === 1
     })
   }
 
@@ -195,7 +193,8 @@ export async function createLogbook(title: string): Promise<DecryptedLogbook> {
           id: serverLb.id,
           encryptedTitle: serverLb.encryptedTitle,
           updatedAt: serverLb.updatedAt,
-          isSynced: 1
+          isSynced: 1,
+          isShared: 0
         })
 
         return {
@@ -216,7 +215,8 @@ export async function createLogbook(title: string): Promise<DecryptedLogbook> {
     id: localId,
     encryptedTitle: encryptedTitleStr,
     updatedAt: now,
-    isSynced: 0
+    isSynced: 0,
+    isShared: 0
   })
 
   await db.syncQueue.put({
diff --git a/server/src/routes/sign.ts b/server/src/routes/sign.ts
index 18f49a8..ea89d1e 100644
--- a/server/src/routes/sign.ts
+++ b/server/src/routes/sign.ts
@@ -61,6 +61,7 @@ async function getLogbookWithAccess(logbookId: string, userId: string) {
 }
 
 function hasWriteAccess(access: { isOwner: boolean; collaboration?: { role: string } | null }) {
+  // Intentional (HYBRID-ELECTRONIC-SIGNATURES.md §2.1): owner OR WRITE collaborator may sign entries.
   return access.isOwner || access.collaboration?.role === 'WRITE'
 }
 
@@ -106,6 +107,7 @@ async function isAuthorizedSigner(
   role: 'skipper' | 'crew'
 ): Promise<boolean> {
   if (role === 'skipper') {
+    // Skipper signing: owner or WRITE collaborator (design §2.1), using their own passkey.
     if (signerUserId === ownerUserId) return true
     const collaboration = await prisma.collaboration.findUnique({
       where: {