Markus, please try to specify encoding for assembly plugin in the following way (also do not forget about "-Dfile.encoding=UTF-8"):
Let me explain why this is required. ZIP is old enought format and initialli it did not know anything about Unicode encodings. Later ZIP specification was extended and support for Unicode and LEF flag was added. The problem is that not all software understand LEF flag. I can surprise you Windows exlorer still do not understand LEF flag. As I know - there is no problems with LEF at linux and MacOS. Windows is still read ZIP assuming that it contain file names encoded with non-Unicode encoding, using MSDOS encoding corresponding to user locale. So if I want to be able to see cyrilyc file names in ZIP files at Windows I need to pack ZIP using CP866 encoding. Not sure exactly, but I think you need to use CP850 encoding to be able to see file names with symbols from german abc.
In the case if you set only "-Dfile.encoding=UTF-8" flag without specifying encoding as in my example - you will be able to see "German umlauts" using archiver which understand LEF flag 7-Zip is one of such archivers.
I think this will solve all your problems with encoding as it did for me.