REST API Issue

I'm trying to use the CreateUser REST API in ADManager Plus via a URL-based request.
This works fine:
https://adad.mydomain.net:8080/RestAPI/CreateUser?domainName=ad.mydomain.com&AuthToken=MYTOKEN&PRODUCT_NAME=MODULE_NAME&inputFormat=[ {    "givenName": "John",    "initials": "mat",       "password": "Martin@12",    "templateName": "User Creation with basic Attributes"  },]
But when I add the sn (surname) attribute like this:
https://adad.mydomain.net:8080/RestAPI/CreateUser?domainName=ad.mydomain.com&AuthToken=MYTOKEN&PRODUCT_NAME=MODULE_NAME&inputFormat=[ {    "givenName": "John",    "initials": "mat",       "password": "Martin@12", "sn": "Martin",   "templateName": "User Creation with basic Attributes"  },]
I get the following error:
{
  "SEVERITY": "SEVERE",
  "STATUS_MESSAGE": "Unable to process the request. Please make sure the inputs are valid JSON.",
  "ERROR_CODE": "00001113"
}
What's strange is that attributes like givenName, displayName, and initials work fine, but adding sn attribute causes this error.
Its not Only about sn some other attributes have this problem too .
SOS