ANother OWA Problem

Here is the Sit. I have a Exchange 5.5 with Sp3 on a NT 4.0 box behind a firewall. I have some users who access the OWA from without a problem. Mostly dial-up ISP users. The i have 4 comcast cable modem users who get the initial logon screen where they enter their username and enter. After they enter a white screen appears and nothing after that. What could be the problem?
Thak you for your help!