So, after more digging, I found the problem doesn't depend on the
amount of songs in the playlist. But I found a way how to easily reproduce
the problem:
Add music files to Cynthiune, via opening a m3u or other playlist file, which
contains songs with an unsupported extension. I for example had a m3u
file, which pointed to a bunch of m4a audio files. When loading such
playlist, then in the Cynthiune Playlist viewer, each file got an entry in the
list stating something about a unknown file format.
Now my solution is, when loading a playlist, then check the extensions
of the music files referenced there, whether they are actually supported,
and if not, then just skip that file in question. See attached patch, which
fixed my problem for me.
Is that sane, or someone with a better idea how to fix that?