It starts to be a pretty serious problem. Now Syncthing deleted one of my files on the tablet, probably while synchronizing it from the PC.
I have edited the file on the PC, and the tablet started showing Out-of-sync status of the folder. The tablet Web GUI stated that it cannot replace a non-existing file. Indeed, the original file was deleted, but there was left the corresponding .synthing.*.tmp file with the changes coming from the PC.
I’m not aware of having set any non-standard settings. I’ll keep tracking down the cause, but now I’m really worried.