I had the exact same thing happen to me 4 months ago. I was running an SMQV ( no chance of mis-seated dials) into my 411a on Block 19. Nothing had changed since last time I used the system successfully. I was getting plenty of RF strength, when I fired up my transmitter (no RF when transmitter was off) but the pilot tone was blinking and no audio was passed. I did bypass the pilot, just to see and got noise.
As with all digital, turning it off and on again was my first instinct. Luckily that did the trick. The pilot symbol was solid and audio passed, I haven't had the issue since, but I may try to recreate the problem, per Larry's suggestion, so I don't get burned on set some day. I hate phantom issues of intermittence.