When using rlm_rest with body = json and the resulting data structure is 1024 bytes long (and probably multiples of 1024 as well, though we haven't tested that), the resulting json structure misses its final }. This is most likely an off-by-one somewhere in the code.
body = json
Easiest way to reproduce is probably setting up an SSID that uses MAC-based authentication, then add some exactly the right amount of garbage to the request before calling the rest module (may take a few tries).
Could you try current v3.0.x HEAD.
Tried it with a JSON-body of 1024 bytes at commit 04abc06, the fix seems to work. Haven't tested it with the master-branch, I wouldn't be surprised if that had the same problem.
Almost certainly, just wanted to make sure the fix was correct before cherry-pick across. Thanks for testing.
Just tested it with master at commit c5b43ae: that works too (and it failed previously)