5DIII locks up with Kenko DGX Pro 1.4 and some lenses?

I am having lock-up problems using a Kenko DGX PRO 300 1.4x converter with a Canon 5D III body.

Specifically, if I use the converter with a 70-300L, all works fine (even the AF). However, if I use a 100mm L macro lens, the camera hangs after taking a shot and can only be recovered by removing the battery. A 5D mark II with the same setup works fine.

Has anyone else seen this problem and found a workaround, or is anyone successfully using the DGX PRO converter in this way?

Has anyone tried modifying the converter like this to just pass-through the camera-lens signals, bypassing the internal chip that modifies the reported aperture?

This is particularly annoying as I don't even have the option to use a Canon converter with these lenses...

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