I bet that there is no bug in the program.
If you can select the field name "TITEL" from the list then you have created that non-standard field before. You should reset the list of fields.
Also, I think that right now you have files with 2 fields:
TITLE
and
TITEL
Check the extended tags for that.
You can salvage the data from TITEL into TITLE with the function Convert>Tag-Tag for TITLE:
Format string: %titel%
Then delete the field TITEL from the extended tags list.
The impression that the modifcation that you applied did not work comes from the fact that you usually see the field TITLE. But as you modified TITEL the modifications are not displayed, the original field TITLE stays the way it was.