This was added at the beginning or end of the X3 .htaccess file? Try to change the rule location? There are several reasons that could happen, and yes it could be related to the X3 rules. You will find many topics on about this on Google search "redirect appends old url to new url", but an easier way to solve this is to use X3's own link/redirect mechanism:
- Delete the rule from .htaccess.
- Create a NEW page in X3 with the OLD URL /Ueber/ (empty content).
- HIDE the new /Ueber/ folder (click the hide button).
- Go to /Ueber/ page Settings > Link > Link URL, set it to /ueber/.
X3 app will now redirect /Ueber/ to /ueber/ without depending on .htaccess rule.