I am hoping, but not for your sake, that someone has had this happen and has a solution:
Upgraded my 5d3 firmware about 2 weeks ago, went out to shoot a wedding this weekend and my Vello batter grip and camera are acting all sorts of crazy. The camera just starts burst continuous shooting until it fills the buffer, waits, then keeps going. It doesnt stop! The only way it stops is if I turn the camera or grip power off. Its crazy.
Yes, it worked fine before the upgrade. With genuine canon batteries and with 2 sets of aftermarkets. Never an issue, camera even gave me level readings of the batteries.
Shutter button doesnt take the picture, but it will focus? Sometimes I could get it to stop shooting in burst long enough to test that but if I got it to stop shooting, the main shutter on the camera didnt work!
Very strange but I am fearing the firmware didnt play nice with this grip. Anyone have this issue or a suggestion?!