Hello @MarcelK,
I believe your problem is that you are in a headless sever and the matplotlib backend is not set up. Greg explains how to fix that issue here: Error converting .qza to .qzv on cluster .
This person's error was more similar to yours and they said that Gregs solution fixed their issue. Error: QXcbConnection: Could not connect to display .
I am not 100% sure that this is the fix but try it and if it doesn't work let me know and we will try to brainstorm. Activate your qiime2 conda env and then run the env command and share the results. It will give us a better idea of what is happening. 
hope that helps!
Chloe 