I had this happen a few weeks ago when that yellow box popped up about device in use... My rendering was missing Midi instruments and contained only RealDrums. A reset of Factory Settings, reboot, and a couple of other things did NOT work. I got rid of it by changing my dxi synth to something else than what I was using (Coyote Forte) and then back again.

Try changing your Midi Output Driver to MS GS Wavtable. As long as the Use VST/DXi is checked, the Output Driver is irrelevant. The key is to get rid of that yellow box. Check this link out http://www.pgmusic.com/techfaq20.htm#76

What dxi synth are you using?

R