Taz,
I encountered similar issues with ReadyAPI and SoapUI when I had special characters or encoding mismatches in my request data. Especially when the request data are exchanged via MS Teams or similar tools, unexpected characters may be inserted.
Postman cleaned the data and sent it modified, so no issue appeared, whereas ReadyAPI tried to use exactly what I inserted and ended up in error.
I would recommend double-checking your request data (encoding, invisible special chars, etc.) Once I removed the special chars, ReadyAPI worked fine.