So two things here,
The first sounds like it is in the province of your membership plugin, as that is where the message is coming from, disallowing access to the page. I have no clue how that plugin works of course but others I have seen add a permission to view section on the actual page details when editing the page. Have you looked there? And if that is not the place surely there must be something in the membership plugin’s setup where you set this stuff up?
And this is stopping access to the forum admin? Correct? It sounds like during the upgrade, somehow your forum permissions have become scrambled. You were updating a very old system! I would suggest that you briefly make the forum admin available and go to the Permissions section and run the task to reset permissions. This does mean you will need to set them up again but that it a fairly simple, if annoying, task.
Actually before dong that – try a less annoying option just in case. Go to the toolbox > housekeeping panel and run the task to Reset Auths Cache. It might possibly be all you need…