After a couple of days I discovered there were problems during synchronizing my wife's files (belonging to user my_wife). Whenever a file was synced from her Win10 machine to her OMV5 folder the owner was changed from my_wife to me so that she could not change them any more. I don't know much about the concept but I'm pretty sure this was caused by Syncthing running with PUID me and not being able to decide between different owners.
My wife creates a file on OMV so this is owned by user my_wife and she can modify it as she wants. This file is now synced to the Win machine and she can modify it again. Now this file is synced back to OMV and while this the owner is changed so she can access and change but not save it back.
chown'ing folders+files to her was no solution because with this Syncthing (running with my PUID) had no permission for her stuff any more.
After reading, guessing and trying a lot with Linux permissions and Syncthing manuals I ended up starting a second Syncthing container running with her PUID for her stuff only (take care about the ports and change them to something else but :8384!).
Now everything's running fine.
But please let me know if you know a simpler way