SBS 2011 - Certificate error when attempting Remote Desktop to server or clients via RWA

Hi all. Here’s my problem.

I’ve got SBS 2011 using a 3rd-party SSL cert, and initial login to the RWA site works just fine.

But then when I try to Remote Desktop into the SBS server or any of the client systems on the network, I get an error referencing the name of the specific remote system on the local network and it tells me that system's cert isn’t trusted. (see image)
Image may be NSFW.
Clik here to view.
I’ve tried viewing the cert and installing it into Trusted Root Certification Authorities on the client per the prompt, but that doesn’t help—the same thing happens again the next time I log in.

I know that I can get into Remote Desktop despite the error, and I see there's even an option to supress the error, but I'd really like to fix whatever the underlying issue is.

Can anyone help?