I'm also having issues with my add-on domain. I believe the server load was under 4 when I added it, but I may be wrong.
Currently the Add-on Domains page in Cpanel shows "No Addon Domains are configured", but trying to add my domain results in an error saying it already exists.
Username: acox
Domain: acox.org
Addon Domain: nastyballs.com
Any help would be appreciated.
Cheers!