Alright, bug was caused by my name which includes a non ASCII letter (german 'ü'). That wouldn't have been a problem, if i didnt use the microsoft account from my university which sets the user name to my real name. Thus in a lot of cases the path to certain folders included a non ASCII letter which Matlab certainly didnt like.
Solution for me was to reinstall windows and to not use my Microsoft account given by the university. Another solution that should work is adding another user - which i sadly wasn't able to due to OS restrictions apparently set by the faculty, that provides the license for windows.
Atleast now Matlab and Simulink work as intended. Hope that helps if someone has a similiar issue.
PS: Something that i should have mention is that the extraction of the Matlab download file also didn't work correctly and i had to download and extract it on another pc and then copy it to my own pc. Should have stopped there, but i guess i'm more of an experimental scientist.
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