Access denied to all non-forum posts + won't disable via module administration

I have a recently upgraded 5.1 site which we'd like to put private forums and moderators on. When we install forum access, non-admins get access denied to all non-forum posts (most of them are cck content types), and some of our views blocks show content from a year ago (although I think this might be cached blocks shown to anon users, so probably exactly the same issue).

Then when we try to disable forum_access in modules administration, unticking the box works, but clicking "submit" leads to a blank page, and going back to module admin shows it as still enabled.

Our solution to this has been to manually disable forum access in system table, then empty the acl and node_access tables, putting the single realm all, grant_view 1 line back in.

I'm not 100% sure this is an issue with forum_access - we used to have strange behaviour with TAC before as well. But that was with 4.6 and I understand there's been major changes with node access arbitrator and stuff.

Some basic information about the site:

13,000 nodes
130,000 comments
six vocabularies (inc one large freetagging vocabulary)
only forum posts get attached to the forums vocabulary - not available to other content types.
Forum posts do not get placed in any other vocabularies.

The forums and private forum appear to work fine in terms of permissions, it's everything else that gets affected.

We've had to manually restore node access at least once before, so it might be messed up (or something related), but this combined with not being able to disable the module got me to stick it here.

There's no errors in the drupal logs relating to it.

I have a copy of the site running on the same server that we used for a dry-run of the update - almost the same db. So could install forum_access on that to give more detailed information about the changes it's making.

Any help much appreciated!