I have now created an article (google translated) about the current problem - there you will also find an illustrated workaround on how you can continue playing on 4played game servers in the current situation:

https://news-4played-de.translate.goog/games/counter-strike-source-unspielbar-steam ...

I was about to open a topic, glad someonelse did that. (not glad about the update from CSS that messed up the servers...)