Accessing ACP modules while testing user permissions returns a General Error

Testing user permission is supposed to temporarily 'deny' access to ACP modules except General.
However, if one keeps a tab opened on an ACP module and triggers the test of some user's permission, when he gets back to the tab with ACP module and refreshes it, not a proper error message by phpBB is showed but a General Error - Not accessible module one instead, like in the attached image.