Cannot Restore Data for Signal Messenger 4.16 with Titanium Backup
I side-stepped the issue:

Turn on backups on old phone (the seem to happen daily)
When restoring, place the .backup file in the sdcard root diretory. Putting it under sdcard/Signal/backups will have the file NOT be read, and no error message given, either.

The 2nd point took me hours to work out.