Had same problem. Happened all at once whether or not I used a 600 EX as the master or ST-E3-RT. I figured everything couldn't have broke at once. I switched all four 600's over to optical and fired a couple shots using one of the flashes as the master. I didn't like working this way so I turned off everything in my house that I thought could transmit RF, wi-fi router, smart TVs, AppleTV boxes etc. then switched the 600's and ST transmitter back to radio control. After that the 600-RTs and ST-E3-RT worked perfect. When I turned on the wi-fi router and all the other stuff the flash and transmitter stopped linking again. When I have more time I'll experiment and see if I can find the specific device that's causing the problem. Will also mess around with choosing other channels on the flash and transmitter. I'm not sure if going to optical and back needs to be done to reset everything in the flashes after they get corrupted by whatever is messing them up. Hope this works for you.
