Issues with files and folders with names containing umlauts
macOS and Linux use different UTF-8 styles. convmv can convert between those two with the options --nfc / --nfd. On your linux based NAS do this:

convmv -r -f UTF-8 -t UTF-8 --nfc  .


From the convmv man page:


  HFS+ on OS X / Darwin
  Linux and (most?) other Unix-like operating systems use the so called normalization form C (NFC) for its UTF-8 encoding by default but do not enforce this. HFS+ on the Macintosh OS enforces normalization form D (NFD), where a few characters are encoded in a different way. On OS X it's not possible to create NFC UTF-8 filenames because this is prevented at filesystem layer. On HFS+ filenames are internally stored in UTF-16 and when converted back to UTF-8 (because the Unix based OS can't deal with UTF-16 directly), NFD is created for whatever reason. See http://developer.apple.com/qa/qa2001/qa1173.html for defails. I think it was a very bad idea and breaks many things under OS X which expect a normal POSIX conforming system. Anywhere else convmv is able to convert files from NFC to NFD or vice versa which makes interoperability with such systems a lot easier.


Btw: This only happens when you transfer your files from a HFS+ partition. APFS doesn't enforce NFD anymore.