When I have a 430EX II connected to a 5D Mark III the menu button fires the flash

[MLeitch]

My Canon 5D Mark III works fine until I connect  my Speedlite 430EX II.  Here's the exact procedure for duplicating the issue:

Power on 5D MkIII

Power on 430EX II

Press Menu button - Flash fires and no menu comes up

Press Menu button again - Nothing happens

In fact no matter what button I press I can't get anything to come up on the back screen of the 5D.

More info:

Even when I power down the 430EX II the menu is still unavailable.

The only way to get the menu back is to power down the 5D MkIII and power back up again.  I can still leave the 430EX II connected just not powered up.

If I do power up the 430EX II it immediately fires the flash and the back screen turns off.

I had this issue using firmware 1.2.3 and having just today upgraded to 1.3.3 I am still having this issue.

Has anyone seen this issue before and are there any useful suggestions?

[MLeitch]

I've now done some additional research on this.  The problem appears to be with the Canon 5D Mark III and not the Speedlite 430EX II.  

I attached my ST-E2 Transmitter and the 5D does the same thing with it attached.  Also I took both the 430EXII and the ST-E2 transmitter and tried the same thing on my other camera.  It works as expected with both.  

So it's clear that the issue lies with the 5D.