Hi Sebastian,
Upgrade your project to 9.2 or the latest version of Mendix. The 9.1 has bug with permissions when deleting.
Could it be that you do not have access to change the all the references but have only read rights instead of read write rights? I would double check those to make sure you are allowed to change it to empty.
Regards,
Ronald
Hey,
I faced this issue with my project and there is two reasons for that :
first : Please check the user role that trying to delete has the allow delete option at all entities related .
second : Change the delete option to a MF like the bellow :
after that I am sure that will works with you .