Users Cannot Logon Unless

I am hoping for some help. I have the Opnefire server up and running on a Windows 2003 server in an AD domain. The client software is Spark. The Client OS is a mix of Win2k, XP and Windows 2008. I also have SSO working. Here is the problem.

Users can logon using SSO or User Name and Password with no problem, but only if I sert their user account to allow them to logon to any computer in the domain. Normally I limit which computers a user can logon to. If I do that the user receives an “Invalid user name or password” error when the try to logon to the Openfire server. I have tried adding the Openfire server to the list of computers they are allowed to logon to, but that didn’t help. Allowing the user to logon to all computers on the domain is the only thing that corrected the problem.

Any ideas on what I am missing here?

Thanks in advance.