I have a EF-X500 flash unit and seemingly out of nowhere if I use TTL the flash will fire but I get nothing but black. If you put it on manual and multi it works as it should. I am not sure if it is some sort of sync issue or what. I get the same result when I have it on FP or Front or Rear Curtain. I usually don’t put the shutter above 125th anyway. The only thing I can think of is that it is just not syncing.