This is configuration issue with your vBSEO. vBET doesn't touch at all forums links.
Most probably you had previously set to remove not English characters from URL, and you had to change it because of translated URLs. Which changed your old German URLs and for some reason vBSEO doesn't accept those now with special characters.
Here is your solution:
Set "Define your Custom Character Replacements" in vBSEO CP-> General Settings ->Character Replacements. Set there mappings for all German special characters. I'm sure that if you search in vBSEO forum for it you will find already prepared mapping
Yes - that is what I was writing about. Please ""Define your Custom Character Replacements" and it will be OK