I saw a lot of people had this problem for me that was a problem between mumble and rift i had to launch mumble after rift and that was okay. Btw if this isn't working try to edit the Rift.cfg file that's in the "C:\Users\<username>\AppData\Roaming\RIFT" folder. Look for "ResolutionX" and "ResolutionY" and set them to the proper values for the resolution you want to run. Just make sure it's a valid resolution for your monitor/graphics card to display.