Yes, this seems to be a CrossFire issue. I had the same problem almost a year ago with my HD4870x2, which was later fixed by 11.5 CAP1
. I don't know what caused it to break again.
Playing in windowed mode helps because it doesn't support CrossFire. There used to be an alternative way of disabling CrossFire by disabling Catalyst A.I. in CCC, but it seems that AMD in their unearthly wisdom chose to remove this option from CCC...
While you may find some registry- and file-editing tweaks to disable it still, it may be easier to use a third party tool such as RadeonPro
to do this. (Use at your own risk!)
Once installation and the initial setup is done, open the application window, click on the "Add new profile" icon at the top of the window, select ROLauncher.exe, click on the Tweaks tab and move the MVPUMode slider to its one-quarter setting (Turn off CrossfireX). Just close the window, changes are applied automatically. Keep in mind RadeonPro has to run in the background for custom application settings to be applied.
It seems like the upcoming Catalyst 12.1 release will have customizable application profiles which will even allow you to tweak some CrossFire settings, so it may become possible to run the game without disabling CrossFire entirely. Until then, this is the only solution I found. Perhaps you could contact AMD support and ask them nicely to fix their application profile for Regnum Online...