Thanks to @GarvinHicking (and Thank you to @Chris aswell, even if what you said did not lead me to the right answer this time)

Are you aware of overwriting methods that are marked "internal" and are not supposed to be used in consuming code? [...] Otherwise, the arguments array should be set via the "injectConfigurationManager" method. Is that maybe not called in your case?!

"injectConfigurationManager()" was not being called since, in my case, an Overwrite existed in the AbstractBaseController of the Extension.

I commented out / deleted that Overwrite, likely a leftover from a earlier TYPO3 version, and the error went away.