Good news everyone!
I found the problem for me. The issue seemed to be the connection between the fans and the Y-Splitter to the pump unit. They were a little bit lose but after squeezing the connection together both fans are working properly again. It turned out this connection is not very robust and can easily get lose when you work on the pc hardware like cable management (I changed the psu before).
Hope this makes it for you too guys!:!: