Holy nightmare Batman, almost had me going there.
http://serveripaddress:2086/command/
Ooops! I lost your webpage [./command/] and I don't know where it went!!??!!
Please forgive me
http://serveripaddress:2086/command
You get the left-hand navigation frame of Cpanel 3. Although some links do not work and it's much more limited than version 5, you still need to have &root& access or SSH permission.
Have a look in: Account Functions & Manage Shell Access, to see if the Client is listed. Must be some logical reason why they got in, but I know not what.
