I have about 500 tracks in the format "01 - filename". The tags Titel and Track are empty. So I want to use the convert filename option with "%track% - %titel%" However, the tag track is correctly updated with the correct value but the title is not changed.
You could try using %title% (not %titel% like the german syntax).
The complete format string would look like this: %track% - %title%
And if you want to remove the no more needed TITEL tag, you can load and select your 500 tracks, press ALT + T and delete the TITEL tag (not the TITLE which is the correct one).