This is a bug in Openlitespeed 1.8.3.1 and their proxy handling - they are sending a duplicate \r\n
which means that one of the request headers ends up the request body and breaks the JSON parsing. We are working on a reproducible demo of the issue then we will raise it with OLS.
I'd suggest changing the web server on your control panel server to Apache or commercial Litespeed as a quick fix.