Remote Desktop Web Connection remaining issue

I cannot connect to my TSWeb Remote Desktop Web Connection on a Windows XP
Pro system from a particular computer. Though I can connect to it through
several other computers, all of which are remote.

The TSWeb page pops up fine. But when I click the Connect button I get:

The client could not connect to the remote computer. Remote connections
might not be enabled or the computer might be too busy to accept new
connections.It is also possible that network problems are preventing your
connection.Please try connecting later. If the problem continues to occur,
contact your administrator.

I initiate the connection just like I would from any other computer:

http://<ipaddr>/TSWeb

I know this computer that I am initiating the connection from is behind a
complex firewall with several routers. If I try to connect through a port
such as http://<ipaddr>:<specificPort>/TSWeb, it works from all my other
computers, but the TSWeb web page cannot be displayed on this problem
computer.

Any ideas? Anything to try? Laplink Everywhere works from this computer.

Thanks,

Jim M