.pdf files do not work in assignement

If a pdf file is added as a resource to an assignement students can not open the pdf. This is rather inconvenient for schools.

There is no error during the upload. But when i switch to a student i can not open the pdf in teams / in the web or download it. Oops there went something wrong is all i get as an error.
I tried adding a file which is in the same team.
I tried adding a pdf which is in onedrive.
i tried adding a pdf from "this computer"
Same effect.

How can this be fixed?