You should report the bug to Firestorm and
Kokua Dayturn teams...
My guess is that they messed up (or failed) the backport of the new coroutine-based HTTP uploads and fetches: the OpenSim (viewer-side bakes) specific code was not any more in LL's viewer and it therefore had to be rewritten with coroutines, which I properly did in my viewer (while Singularity did not yet backport the coroutine HTTP stuff at all, so they are still using the old baking code).
Another possibility would be an incompatibility in the JPEG2000 decoder (Firestorm is using KDU, which is known to have issues with OpenJPEG encoding in other TPVs), but I don't think Kokua is using KDU, so it's probably not the issue at hand (though, some new OpenJPEG versions do have issues too, and my viewer and Singularity are using an old OpenJPEG that is known to work properly).