Hi experts,
copying the apps folder obviously helped. I saved the "old" one, copied the complete folder from the production system to the test system and then replaced the Ajax folders again by the "old" one's because they seem to be complete and I didn't want to risk a version mismatch.
Then I renamed server0 again so it'll be rebuilt.
And the result: everything seems to work now - NWA, SystemInfo, Portal itself...
So for next time I'll control the file system backup twice before I do actions like patching or the like...
Sometimes you miss the forest for the trees when the search takes too long.
Many many thanks for your suggestions and help!
Thanks and best wishes
Marie