This FAQ page is only relevant to versions of MainBoss Advanced before Version 3.3.
In the MainBoss Installation and Administration Guide, go to the section called "Installing the @Requests Service", particularly the subsection titled "What Name Do You Specify for Running the Service". It lists a number of rules that must apply to the login name you specify.
One common problem is not entering a fully qualified login name. The login must be entered as
if the name is part of a domain. Otherwise, the name must be entered as
where COMPUTER is the name of the computer.