I've solved this one, but just in case anyone else had the same issue.
If you try and update the adv attachments addon to v3.x before updating wpforo you will get a critical error that requires you to use the WP recovery mode (an email will be sent to you from your site with the link) to disable the offending plugin before things work again.
Instead. DISABLE Adv Attachments plugin. Then update wpForo, then reinstall and re-activate Adv Attachments v3.x
And then 🤞 all works ok 😀