The JSON version was updated starting Core 0.28, likely a benign update of the underlying library. This broke Armory's handling.

In the previous JSON version, the "error" field is mandatory, regardless of whether the request was successful or not. Armory 0.96.x treats replies as malformed if the field is missing, which the new JSON version can omit.

Regarding your options with 0.96.5, you can either apply a patch yourself to 0.96.5, stick to Core 0.27.x, or run Armory without Core RPC support. If you're more patient, 0.97. I'm hoping to release it before the end of summer.