Please note, that the old versions of wpForo Advanced Attachments addon are not compatible with the just released WordPress 5.9 and higher versions. Please go to Dashboard > Plugins admin page and update the addon to v2.1.2 or higher version.
If the update is not available, it means your license key is expired. Please login with your customer account and purchase the addon with 30% discount to get a new license key and update it.