Just to add to this, My D7100 works with 1.4, 1.7 and 2.0 (III) converters. It is spot on focus on the 1.4 and 1.7, the 2.0 is not a great combination, but I'd hardly blame the camera for that. Its useable, but not a great combo.
Others have pointed to the lens and it does seem to be the likely culprit.
Hope you get it working, as it will be a great joy on the D7100 I think DJ
.. I set out to discover the inventions of God. -John Muir