When I log in with my administrator account I am able to see the Create Content Link in my Organic Group. But when I am logged in with another account - permission to create content for content types is set - the create content link does not appear. I also activated entityreference prepopulate. Does anyone know what the problem could be?
Not sure which permission you mean with the "Permission to create content for content type is set" (consider adding the URL within admin to clarify that, is it
However, it could well be that your issue is because you've not yet completed the "OG Permissions" located with something like
/admin/config/group/permissions/ (and then select the 'edit' link for the group type your issue is about).