Cubase 6:
I press on the solo button of an fx track and the master is muted???
I had this problem with single tracks, but I removed any sends from the problem tracks, saved my project and restarted Cubase… I could then re-apply the sends and everything works as expected…“works as expected” is obviously an understatement 
Is the master supposed to become muted when an Fx Channel is soloed?
I tried clicking the main mute and solo buttons but that did not work…
And you´re sure, the FX track is routed to the master out…?
Yesss that did it thanks Thinkingcap !