A new experiment and clean install after purging ST, deleting ~/config/syncthing. I started with 1.5.0 on both sides and all synced as expected. Adding or deleting files no problems.
I upgraded to 1.6.0-rc4 on one side, and again no problems.
Upgraded the other side to 1.6.0-rc4 and everything is stable and working as expected.
I have start-up console logs from both sides and a screenshot after the successful restart with both sides on 1.6.0-rc4 but I guess if all is working again you won’t need these.
Again, thanks for your help. I assume the changes between -rcs solved the problem described at Database migration 1.6.0~rc2-rc4 (second screenshot).