My guess would be the files are in an unexpected normalization and the file system is doing something helpful but unexpected when we try to correct it. I don’t know how synology handles normalization, might be something to read up on if it can be changed.
If the files were already on there before you started using syncthing, see if moving the files to a different directory (so syncthing gets to create them) has better luck.
(If all this sounds like greek to you (and you don’t actually speak greek), the background is that there are multiple ways to store a character like
Ä which are all “UTF-8” but sometimes can’t coexist in the same place. Syncthing finds a file with the “wrong” encoding and tries to correct it, but there is apparently already something else in place using the “right” encoding too.)