Unfortunately I still have no explaination for that. I tried it again a few days later and it suddenly worked. I thought “Okay, problem solved”, but the next time I wanted to import something to UE4 the same problem occured again. 
Unfortunately I still have no explaination for that. I tried it again a few days later and it suddenly worked. I thought “Okay, problem solved”, but the next time I wanted to import something to UE4 the same problem occured again.