Can't skip tracks or play new ones, only pause, forward/rewind works

[EpicLPer]

First of all thanks for this project :)

I'm having a few issues however. Things work somewhat so far, I can play a song and it also plays to the end as it should.
However I can't skip nor play a new song, pausing sometimes is a bit iffy, skipping forward/backwards also works fine. Plexamp (on iOS) shows the progress of the song, but starting a new one also shows the progress of the old song on the new song.

I'm using HiFiBerry OS as the receiver, if that matters. Thanks for your help already!

EDIT: On the Plex Webinterface it only works when I press "Stop" on each track first, then play the next one. However there the progress doesn't work at all.