The issue is caused by a conflict of our Javascript and any other Javascript framework deployed in the monitored application.
In particular that both our monitoring and the monitored framework are making similar API calls, and the framework in the application can override the JSON API we use.
This leads to creation of unnecessary escape characters in the JSON payload, you would see something like this in the log error:
"[{\\\"id\\\": \\\"6a1df92274bf4766b065cc8e95125b6d\\\