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.
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.stackexch ange.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.