Can’t connect to official cs:go servers due to packet loss

I can't connect to the official valve servers anymore. Community servers still work, but when trying to connect to both competitive or casual servers, I get a weird loadscreen without any texts, my packet loss suddenly rises to 99%, and the console gives out the following error over and over:

<Server IP>:corrupted packet -<Some number?> at 0

Here is a picture of the loadscreen and the console:

(There are no texts or pictures behind the console. The relevant parts are highlighted in green)

I opened all ports listed here and even tried reinstalling the game, but nothing seemed to help.