Hello Armin,
It's probably too late for you because you've solved your problem in other ways. But there may be other users with the same problem. There's a simple way: Mark all the images in which you want to delete all keywords. Then open the 'Metadata (IPTC) Template' with Crtl + I. Now first click on 'Clear' at the bottom left and then check whether all fields are actually empty and none of the boxes are selected. Then tick the box at 'Keywords:', but only there and not the box next to it with the '+'! The text box for keywords must be empty. Now click on 'Apply Template to Selected'. This step only deletes the keywords, but leaves all other metadata untouched (for example, Copyright Info).
Erich