For more than a year now I was able to POST with body of:

{"body":"There is a sales order in the system, matching your invoice request (invoice number 112316 ). However, the order has not been fulfilled and the invoice has not been generated yet."}

It now gives an invalid_json error even though the json is valid according to json validator. 

I now have to POST this body without the number in it:

{"body":"There is a sales order in the system matching your invoice request however it has not been fulfilled or invoiced yet."}