I have backed up and restored Signal with Titanium for years without any incident, up until now. The other day I was encountering a boot loop playing around with an Xposed module. I did a re-flash of the ROM to regain access and then backed up my data with Titanium as usual. After a full wipe and re-flash all data was restored correctly (even for system apps), except Signal which force-closes instantly every time I open it. I've read about false UID re-assignments and tried the Fix Permissions app without any result.
Fellow member @Josh experienced the same issue and attributed it to changes in the most recent Signal version 4.16 which pointed me into the right direction to find a workaround. I installed version 4.15.5 from here and then restored data only from Titanium. Now Signal will start normally again, but my messages from about the last 2 weeks are missing. Of course this can only be considered a temporary solution. I wonder what can cause this kind of behavior in Titanium, and if there are any similar backup solutions available that do work with Signal.
Update: At last, the stable released yesterday (4.17.5) has finally gotten the long-awaited backup option:
For rooted users, it would still be easier to backup and restore all apps in one batch after a ROM switch. Unfortunately, the upgrade from 4.15.5 caused Signal to crash instantly upon opening. All I could do was downgrade and export a plaintext backup which means I have now lost all media and group messages. Note that when you import the plaintext backup, you might need to use an earlier version. I couldn't restore with 4.17.5 since the app would always crash (10+ MB file). Only after going back to 4.15.5 the plaintext import was successful. Upgrading afterward went without problems.