I see a suspected mismatch of ID types in the code. syntbase uses GetRemoteVolumeFromID which takes a RemoteVolume table ID, but is given a Fileset table ID from lasttempfileid which uses GetIncompleteFilesets. This could explain the weirdness when it targets dindex and dblock files. The variable isn’t used in the actual synthetic filelist creation but might suppress it inadvertently, in addition to causing hard-to-answer questions.
Found this while looking at my own log warnings of this sort while wondering if there’s any reasonable way of digging the last few files out of the partially completed backup here where it’s unclear it will do more backups.