Dear Stefan
Did you solve the problem eventually?
I've just tested master branch on my Fedora 32 machine and it works
fine.
I don't use MIDI input actually, but I have a little MIDI keyboard just
for testing purposes.
A couple of years ago I wrote a stub of a wiki page here:
https://github.com/frescobaldi/frescobaldi/wiki/MIDI-input-on-Linux
Please read it and see if it can help you to get started.
Frescobaldi requires PortMidi for both MIDI input and playback (see the
INSTALL file in the source code).
Your python3 installation should have installed one of the modules
providing python bindings for PortMidi.
For example:
https://packages.ubuntu.com/bionic/python-pypm
Which version of Ubuntu are you using?