I am also getting this. My user does have full access to the folder - and the file!
MusicBee v3.3.7491.38139D (Win10.0), 28 dec 2020 16:20:
System.UnauthorizedAccessException: Access to the path 'C:\Users\[USERNAME]\AppData\Roaming\MusicBee\Tag Backups\.Master Tag Index.mbi' is denied.
at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share)
at MusicBeePlugin.BackupIndex.saveBackup(String backupName, String statusbarText, Boolean isAutocreatedParam)
at MusicBeePlugin.Plugin.regularAutobackup(Object state)
at System.Threading.TimerQueueTimer.CallCallbackInContext(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.TimerQueueTimer.CallCallback()
at System.Threading.TimerQueueTimer.Fire()
at System.Threading.TimerQueue.FireNextTimers()
at System.Threading.TimerQueue.AppDomainTimerCallback(Int32 id)