We have just seen this exact problem arise again 5 days ago with the New proxy.
All of a sudden the proxy seems to 'modify' the body of the POSTed JSON object and the service is not able to process it anymore. It was working fine up until end of last week.
GET calls work fine through the PROXY but POST calls are broken when made through the app.
Using the TEST function in Appery on the service, both GET & POST work correctly.
Seems like this bug has appeared again?