Hey there,
thanks for the effort everyone.
The problem also occured with 2.9.4. I upgraded to the newest beta because it includes a new template and I suspected it might integrate better into enfold.

I have absolutely no idea what causes this 404 problem for subscribers. As this seems to be an issue specific to our page, I will stick with my workaround for now and I think it’s not worth wasting your resources. So far, it works without any issues. Though, I will let you know once I found the root cause for this problem.

Meanwhile I found a workaround for my problem. I have created a new role based on the subscriber role (FVIM-Mitglied) and added the capability for editing pages. Once the user role has this capability, users cann access the members page. However, they are able to submit pages for revision and can access theme options from the frontend. I worked around that by denying access to the dashboard via another plugin.