Thank you for your tips! Indeed the interaction with the repository is have had some quite confusing problems before, too. I tried zulu with no success. Also, I could not get spoon to produce any debug output. Downgrading the Java installation is always something you would like to avoid as long as possible.
The workaround for now will be to create the needed entries in the repository either using spoon but without having the repository connected or directly to the file system.