AMD Radeon Software auto-updates.
When updating Radeon Software to Version 15.12 it self installs an Application called AMD Quick Stream Technologies. (Box is checked by default but can be unchecked. )
The application auto-starts with the Radeon Software on Windows Startup and blocks the connection to the Realm host server leaving me unable to connect to any Realms. The error displayed is as follows when attempting to connect to the Realm :
"IOException: An existing connection was forcibly closed by the remote host "
After disabling the application, I was able to connect to the Realm without further incident.
Maybe just post a warning to others regarding the issue.
Hope I have saved others some time troubleshooting.
Cheers
GoSlowJoe
Please report that to AMD.
Edit: Yes, thanks for your report. Forwarded that information to @unknown who's helping in IRC #minecrafthelp