Dear @hardy,
thanks for the detailed reply. We were using HBV 5.2.1 for a long time on Appengine without any problems. So it seems even if not officially supported on App Engine, it was quite stable. It would be shame if such a great library would stop being supported, especially when alternative solutions for ResourceBundling are available.

Hopefully a compromise solution will be found.
Meanwhile we have switched to 5.2.1.

Thanks