Library sync causes server error

Hi all,

since some days the Seafile client on all computers cannot sync any libraries anymore. No matter whether it’s a newly created or an existing one.
The client then shows the error message: "Information on downloading the package sources was not available: Please server error " . While “Please server error” (in German: “Bitte Serverfehler”) is the funniest “reason”.

But what is the problem with the clients or the server?
I’m using Seafile client 7.0.4 on Windows 10 and Seafile server 7.0.4 on a RPi.
Server update to 7.0.5 failed, which I posted here: Upgrade from Seafile server 7.0.4 to 7.0.5 failes