Comment 7 for bug 1954453

If files with special characters like Umlauts (öä.. etc) are displayed correctly when stored on your harddrive (or SSD), I guess the issue is either, that the problematic filenames contain a byte sequence that isn't valid UTF, or something is strange/wrong with the way how your Ubuntu connects to the NAS.

But if you are experiencing this also with local files stored on your HDD/SSD, or while typing in the terminal / text editor, it is more likely a problem with your locales.

See for example the thread below and check if your locales appear to be messed up.

https://unix.stackexchange.com/questions/425929/problems-with-umlauts-cant-type-in-terminal-cant-open-files

If so, try to reconfigure them. ('sudo dpkg-reconfigure locales' should do the trick on Ubuntu)

Anyhow, if the problem also occurs in your file manager, I would say it is almost certainly not a bug in Mixxx.