Can you post screenshots from the Web GUI that show exactly the error messages that you encounter?
I’ve probably had a similar issue last year in https://forum.syncthing.net/t/failed-items-and-access-is-denied-in-a-receive-encrypted-folder/17479. Unforunately, I’ve stopped experimenting with Receive Encrypted folders after that, because not being able to sync multiple files with no clear culprit that would be fixable was a deal breaker for myself.
I did try to experiment with using symlinks in drive root paths pointing at the proper folder, but they didn’t help. I did not try moving the whole folder to a very short path, e.g. C:\u_z, so I’d suggest you try doing just that and see whether there’s any difference.