As mentioned in the thread that was split from this one, I found our issue. It appears we have a task that removed the working directory in order to have a clean Code coverage result. Since the scanner is downloaded in the Sonar task since version 6 it was removed with the working directory and thus was not found later on. We will adapt the pipeline accordingly.
I ran in yet another problem where our OWASP dependency check tried to upload files from the scanner for some reason but was able to resolve it by setting the report directory to a sub directory.