Hi,
It is said connections to an SBS server that requires an ActiveX control which is not supported by mac.
However, after searching further, there is a very old post describing how to remote from mac via RDC together with RWW.
You could take a look at below link and verify if the workaround provided would be helpful.
https://thecomputerperson.wordpress.com/2014/07/27/os-x-mac-microsoft-remote-desktop-8-0-8-and-remote-web-workplace-problem/
Please note: Information posted in the given link is hosted by a third party. Microsoft does not guarantee the accuracy and effectiveness of information.
----Please "Accept as answer" if the reply is helpful-----
Thanks,
Jenny