Is the Yongnuo Canon ST-E2 compatible with the Canon 6D?

Since this involves flash photography I thought I'd ask here. I recently sold my 5D, the original 12.8mp camera. With that body I used my Yongnuo ST-E2 and 420 and 430EX's off camera for a lot of my work. I replaced the 5D with a 6D and tried to use it for some off camera work and every picture at every setting is underexposed. I even dialed in +3.0 exposure compensation and +5.0 compensation trying to get an image that wasn't underexposed. To make sure my ST-E2 was in good working order I grabbed my 15 year old 1Ds and a couple of Speedlites and everything worked perfectly. Any idea what the problem might be? I've gone over the 6D's settings many times but can't find anything that would cause a problem. I also tried my Speedlites on the 6D and every exposure looked well exposed. Can't figure out what the problem is.

It's quite possible that Canon have tweaked the flash interface on the 6D. Maybe even purposefully to make 3rd party flash interfacing more difficult.

 

I know that Nikon have been constantly "updating" their flash protocol with every new camera model. It seems specifically to remove any backward compatibility with their previous film TTL speedlights. I also have a YongNuo TTL flash extender cord that works every time with a D700, but only works sporadically with a D800 and D7200.