Pops and hisses with effects

I have a project with a about ten audio tracks in it with several takes per track, plus a dozen EW Play instruments which are frozen. There are EQs and some reverb and compression on some of the tracks. It's thick, but not overly complex. This is a new computer with nothing on it besides my DAW setup.
 
Now when I play back I often get a loud pop and then hissing, and my audio tracks don't play. I just pause and play again and sometimes they will then play fine, sometimes I get the pop and hiss again. I just keep pausing and playing until it works. Sometimes it only takes 2-3 tries, sometimes a dozen. This never used to happen on my old, less powerful computer. When I disable effects, the problem never happens. The effects I'm using are all either ProChannel EQ or else Sonitus. I've messed around with buffers and things but don't know what I'm doing.
 
Also, now when I open the project, I get an error that says "The following track or buses are currently assigned to a silent hardware output. The items listed below will be silent in your project until their outputs are reassigned to an appropriate hardware output." Then it lists one of my audio tracks. But guess what - not only is that track no different from the others, it plays just fine.
 
What's happening?