The following is what I would try.
If you can get to the admin, make the phorum unavailable
Get a backup :) of everything.
Turn the module off.
Clear/Empty the cache. Verify it has been emptied, or do it manually using the Hosts "panel"
Using phpmyadmin, or another admin tool, run maintenance on the table.
Verify the "Online Users" scripts have not been tampered with.
Re-enable and test.
As a last resort, empty the (Online Users) table (I don't remember what the repercussions of this are, if any.)
Edited 1 time(s). Last edit at 01/23/2018 02:44PM by Scott Finegan.