01 Fault code....on ef500 f4 L IS MK1

Hi i've used both the canon converters on my old ef400 2.8 L IS lens on a 5D3 body with no problems.
I now have a Canon ef500 f4 L IS mk1 and when putting the X2 iii converter on i get the 01 fault code Communication error between lens and camera clean contacts .....has anyone had this problem ?
Everything is ok with no converter attached or even with Canon 1.4 X iii.
I forgot to add it worked fine before updating to firmware version 1.2.3

This doesn't help you much, but the 2xiii does work with the 500 and 5D3 as I used to have one, so you can at least rule out incompatibility.