I experienced a different problem after installing KB300850. I don’t use Avast, but I could no longer open RDS Published applications. (Windows 2008 R2 Remote Desktop Services)
When I logged in on Remote Desktop Services Web Access, I could start the published applications but it suddenly gave me a second login prompt.
After I re-entered my credentials, it kept saying ‘starting’ and the window eventually disappeared.
I’ve tried this on different RDS environments I administer for different customers; same issue on each one of them, so it was definitely a problem with my Windows 8.1 client.
I also tested on a second Windows 8.1 machine with update 3000850 installed. Same issue.
Finally, I tested this on a Windows 7 client and there I had no issues using the published applications.
I decided to remove update 3000850 using Programs and Features (do not use System Restore!) and after rebooting, I could use the RDS Published Apps once again.
I hope you will find this helpful.