It was on a Shipping build and has gone through the whole steam pipeline.
It was compiled using the “Shipping” Setup for the client and server, then uploaded to steam (both, client and server).
The server was running on a remote server in a data-center, the client was then launched with the steam client, just like any other game.
Currently the error is gone, I have nothing changed, the only thing was a clean build of the project (took some time).
I deleted all cache-files before the rebuild.