membership plugin prevents pages from being edited

After activating the plugin on one of the sites of my network, it prevents all pages of that site from being edible: "Edit" of any page results in a blank page. This also applies to the pages like "protected content", "register", etc. As frontend user, everything remain visible. Posts are not affected.

In addition, the right sidebar shows the following error message (see screenshot):

Fatal error: Cannot use object of type WP_Error as array in /srv/www/vhosts/ on line 257

Any ideas...? Thanks!