"error": "Error creating issues in Jira!" (rest cloud API) / xray

[Alexander Brandhaug]

I'm trying to post some testresults from my unit tests to Jira using this endpoint: https://xray.cloud.getxray.app/api/v1/import/execution/nunit/multipart

{
"fields": {
"project": {
"key": "INF"
},
"summary": "EmailAgent automated testcase",
"description": "this testcase is automated and created upon automated test runs",
"issuetype": {
"id": "10258"
}
}
}

the issuetype with id: 10258 is of the type:  Test Execution.

I'm getting 400 HTTP response with the message: 

"error": "Error creating issues in Jira!" which doesn't really make me understand what I'm doing wrong. 

I'm adding the json as a json-file in the form, along with my testresults (XML).