Thanks for your update.
Besides the firewall factor you mentioned, it is also necessary to troubleshoot the remote workstation web browser(IE) and RDP( 6.1 client or higher )installed on the remote
workstation. That's why I'd like you check and upgrade the RDP version and reset the IE settings and register the "Mstscax.dll" dll on that remote workstation.
Regarding other thoughts, here are some general information of RWW for your review reference:
Windows SBS 2008 sends all new users a "Welcome" e-mail that introduces them to the network features including Remote Web Workplace. Before users can access Remote
Web Workplace, network administrators must satisfy the following requirements:
Prerequisites for accessing Remote Web Workplace
RWW in SBS 2008 leverages the TSGateway service that is running on the SBS server to perform the remote desktop connection to the chosen machine. Like RWW,
TSGateway is fully enabled when the IAMW is completed ("Add a Trusted Certificate" must also be completed if you are using a 3rd party SSL certificate). This allows remote
desktop connections to your domain-joined machines through port 443. This is different from RWW in SBS 2003, where you had to open port 4125 through your firewall.
SBS 2008: Introduction to Remote Web Workplace
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