So, I tried everything:
- completely removed VM (all files and database tables); Installed VM new with an empty Shop, made only mandatory changes for the test.
- same with an older VM Version
- disabled every plugin expect VM
- used beez template
- disabled SEO, Cache and other joomla features
-> same Problem!
If I install a clean Joomla and VM on my Server, everything works fine.
So I guess it has something to do with my Joomla.
Going back a Joomla Version from a backup -> same problem.
I have two different Websites, the second was once a copy of the first. Both have this problem.