I am attempting to create a Pardot Prospect on the fly in the back-end using the following command in an Apex class :

String returnedResponseFromPardot = Http_Utility_Pardot.pardotCreateProspect(mapContacts.keySet());

The class is triggered with an action button in a record. It was working at one time. But for some reason I am now getting this error :

09:01:22.0 (166342131)|METHOD_EXIT|[38]|01p2J000004cn7i|HTTPResponseParser.parseLoginResponse(System.HttpResponse)
09:01:22.0 (166357697)|METHOD_EXIT|[65]|01p2J000004cn7j|Http_Utility_Pardot.login(Pardot_Tracking_Configuration__mdt)
09:01:22.0 (166369610)|METHOD_EXIT|[136]|01p2J000004cn7j|Http_Utility_Pardot.pardotCreateProspect(Set<Id>)
09:01:22.0 (166402011)|VARIABLE_SCOPE_BEGIN|[154]|ex|Exception|true|false
09:01:22.0 (166476337)|VARIABLE_ASSIGNMENT|[154]|ex|"common.apex.runtime.impl.ExecutionException: Attempt to de-reference a null object"|0x5f998941
09:01:22.0 (166487094)|STATEMENT_EXECUTE|[154]
09:01:22.0 (166489205)|STATEMENT_EXECUTE|[155]
09:01:22.0 (166492972)|HEAP_ALLOCATE|[155]|Bytes:56
09:01:22.0 (166510285)|SYSTEM_METHOD_ENTRY|[155]|ResponseWrapper.__sfdc_message(String)
09:01:22.0 (166563630)|VARIABLE_ASSIGNMENT|[EXTERNAL]|this|{"isSuccess":false,"message":""}|0x2cb34610
09:01:22.0 (166578984)|VARIABLE_ASSIGNMENT|[EXTERNAL]|value|"Some error occurs in (36 more) ..."
09:01:22.0 (166595013)|VARIABLE_ASSIGNMENT|[4]|this.message|"Some error occurs in (36 more) ..."|0x2cb34610
09:01:22.0 (166605506)|SYSTEM_METHOD_EXIT|[155]|ResponseWrapper.__sfdc_message(String)

Anybody have any idea on what is causing this ? I'm wondering if it may be a fault with the API user which is being used ? Maybe it needs to be re-validated in Pardot...?

If you have any other suggestions I'd love to hear it. The error is so vague I'm having trouble really evaluating it all.

Thank you very much.