Hello,
I am forwarding a bug report from Debian's bug tracker.
This is Debian bug https://bugs.debian.org/798399.
"When trying to open a file which has non latin characters at filename or at
path, a message appears:
Cannot open "/home/user/???????/Fuer_Elise.mid
If I move the same file to another directory which has only latin characters in
the path, then it opens normally.
The open file dialog should be able to parse urf8 characters."
I can confirm this behaviour when the path to the midi file contains German umlauts, pianobooster won't open the file at all. If I move the file to a directory which contains only ascii characters, opening works as expected.
Please remove the dot at the end of the above link to make it work.
or use the correct link here: https://bugs.debian.org/798399
Forwarding: https://github.com/captnfab/PianoBooster/issues/53