After trying alot of renaming which didn’t help I did the following approach:
I looked into my Browser Console and into the Error Codes that appeared there. After some research and a look into the XAMPP Apache error logs I stumbled upon the following error there:
Exception: Required extension GD is not loaded. in C:\\xampp\\htdocs\\kirb [...]
Which I fixed by adding extension=gd to the bottom of the php.ini File in the Apache Control Panel. Which also fixed the Cover Image Problem for the default content in the starterkit-main
Thanks for all the help so far