I searched the net and it looks like that the problem is only with this particular Kenko Teleconverter:
Kenko Teleplus PRO 300 DGX 1.4x AF
Other Kenko products are probably not effected.
It is also possible that the problem is only in some particular version of the firmware, because I've seen reports of some people using the Teleplus PRO 300 DGX 1.4x with 5D mkIII.
Since the problem doesn't effect a large number of customers it is highly unlikely that Kenko would invest any efforts in solving it.
I don't believe also that it is in any way connected to the 5D mkIII firmware version, at least for me, having the last version (1.1.3) doesn't help much.