Found out some more things. I made a short test and recorded the result (vimeo link, see below).
Reaper is running through Jack (Cadence) and the Pulse Audio Bridge is enabled in Cadence. When I minimize the master volume in Reaper and play something through Chromium I get pulse audio xruns when I move the play head in Reaper. The RT CPU usage of that project is ~9% only. Strange thing is that I get RT xruns, but the RT longest block (shown in the Reaper performance meter) stays below the latency of 11ms. And if the pulse audio bridge is off everything is fine -> no xruns. In a new project with just an audio file loaded also no xruns.
So, how can this acutally cause a xrun? Should I post it in the Linux bugs section also?
https://vimeo.com/345774754