I did a little research on this issue and found this
https://laur.uk/2021/03/10/umlaut-special-characters-issue-over-smb-on-mac-and-synology-nas/I haven't tried it myself yet but it looks like a possible solution.
My Mac still throws errors when I import tracks edited with Tag Editor on the Mac with diacritical character in the file name and when I update the track in MC with F6 (which changes the file name according to my rule:
[Album Artist]/[Album] as base path and
/[Disc #]-[Track #] [Name] as file name. MC can still find the tracks but the name doesn't actually change, for example when I try to convert "ö" to "oe". I am probably doing something wrong but it is still a pain. I am going to spend a bit of time with a test album to see if this latest find can solve the issue.
EDIT:
When I create a diacritical character in folder and/or file names within MC there is no problem with them. I can save them with F6 and transfer to rPi and NAS (using Foldersync 2) and there is no problem. However, when I import tracks that have been modified in Tag Editor and contain diacritical characters, like
1-09 Janáček – Mládí - II. Andante sostenuto
a problem arises
Even though the folder and file names appear identical, when I try to F6, I get an error and none of the files are changed. They are still recognized in MC on the Mac but are not recognized on the rpi nor the NAS.
I also couldn't get the rsync to work. I am sure it has to do with the de-composed coding but it is still a pain.