A colleague from work answered the question for me. The answer is clearly PEBKAC... I deleted a module/script which is still being called from the unittests. The module merger.py was deleted.

It was my own PTSD from the last incident that I have to deal with this all over again which prevented me from taking it step-by-step. He showed me how to follow the white rabbit by checking which commit had the first failed unittests. then checked the diffs in the commit where I then screamed "AHHH, I deleted the module which is trying to be imported from"

In hindsight: stress and anxienty are a hinderance when analyzing ones own issues. Little experience and carelessly deleting stuff on the repo without using tools that warn about imports from files you are deleting, are also a bad idea.