Hi Ikcarus, welcome to the forums!
I can confirm that this is indeed an issue with the current Screenly OSE on Raspberry Pi 4. Thank you for raising it. We’ll see about getting it fixed ASAP.
In the meantime you might want to check out this thread [1] since the symptoms are similar and a workaround has been found. Essentially you downgrade the version of celery specified in requirements.txt to a supported version. The latest supported version is 4.4.7.
If you do try this, please let us know how you go so we know whether this workaround is still valid.