Isaac,
You are spot on with the problem.
When I select login I get for a url
http://acusersforum.com/ub/ubbthreads.php?ubb=login&ocu=http%3A%2F%2Facusersforum.com%2Fub%2Fubbthreads.php%3Fubb%3Dcfrm
I get the mod_security error.
But while that url is in the browser I just edit it and remove http
acusersforum.com/ub/ubbthreads.php?ubb=login&ocu=%3A%2F%2Facusersforum.com%2Fub%2Fubbthreads.php%3Fubb%3Dcfrm
It works.
Also this URL works
acusersforum.com/ub/ubbthreads.php?ubb=login
Could this be a temporary fix till the next version?
Not sure at this point in time where to edit and just hardcode the login link to that right now
