Rest API error when trying to get token

This used to work but finding that when trying to get an auth token the following error is returned:

{
    "Status": 400,
    "URL": "/server/authenticationtokens",
    "Method": "POST",
    "Exception": {
        "Name": "ArgumentNullException",
        "Message": "Value cannot be null.\r\nParameter name: value"
    },
    "Stack": "Error reference ID: c3e388ea-aa9c-46cf-8e76-4c72eef82d05",
    "Message": "Value cannot be null.\r\nParameter name: value",
    "IsLoggedToVault": false,
    "IsLoggedToApplication": false,
    "ExceptionName": "ArgumentNullException"
}
This is via /REST/server/authenticationtokens parsing Username, password and VaultGUID via Postman.
So its a rad request but not sure why as I am parsing all the same parameters as before:
{
    "Username": "XXXX",
    "Password": "XXXXXX",
    "VaultGuid": "XXXXXXXX"
}
ect
I have also tried reinstalling REST as well on the server. Anyone know what I may be doing wrong?
Thanks