In a 2012 R2 RDS environment where Web Application Proxy and RD Gateway are used we found thatMac RDP clients could not open RemoteApps after installing the January 2020 Windows Updates KB4534297 & KB4534309.
Mac users were getting errors 0x204 or 0x3000008 when attempting to connect.
Windows RDP clients could connect fine. The main issue was with the previously mentioned updates on the Web Application Proxy server. Uninstalling these updates from the Web Application Proxy server and rebooting allowed Mac connections to again work through
Web Application Proxy.