Editing in the backend not possible

Hello and hello,

unfortunately, our problem has not yet resolved: We simply can not view and edit existing and new entries in the backend because they are no longer displayed. What can this be? In a multi-site installation, only one of two pages is affected, multisite is not officially supported. But another user seems to have the same problem. Can something be done there? We would compensate for the problem solving and the effort.

I can send a screenshot, if you need one.

Many Thanks!