You use wpForo on some irregular server or you have hacker attacks. There is no way to not have HTTP_REFERER. Please provide more info about your server and when you get this error. If this is a log file delete it and check if it's a permanent error. Maybe some hackers tried to access admin functions with bots. wpForo is very secure in this cases but it may generate such error if the referrer is bot or if it's not found.

Nothing irregular, it is a stable server with Litespeed and CloudLinux. I have no idea when this happens, it is in the logs from time to time.

Hackers sure, all the time they try things in all WP installations, not just for me. I use The Shield plugin for all security. Could that be it ?

Thanks

Ok, than it hacker attack issue. Thy try to do something using direct call of WP Dashboard files. You can ignore this note. We'll make some changes in core to not generate PHP Notices when referrer is not found.