I tried compact/repair and several other things before importing all your objects into another db, figuring the db was corrupt but no difference. So I copied the rowsource of Level 1 from Formular Hämatologische Tumore then deleted the combo. Then added a new combo, pasted in the row source, reselected the bound field and problem gone. All I can conclude is that you had one corrupt combo which you copied everywhere, either individually or by way of copying forms.
Spaces and special characters in object names (save for underscore) is not advised.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.