hi they have to have
TUTORIAL: NEW Fortnite THE RIFT Server Set Up and Download / How To Set Up Rift and Modded Server - YouTube
and you have to have everything public.
FN Creative yes, but not UEFN
if you set the UEFN connector as private does this not stop everything
Have you checked who is on you Team group for access and updates.
How about transferring the project control to a new team in the content server