The plugin stopped working in jobs over the weekend and will no longer validate the Jira Project Key and allow further config and use. This is most likely caused by Jira Cloud deprecations for GDPR - issue was seen starting this week - https://community.atlassian.com/t5/Jira-questions/JIRA-REST-API-missing-parameters/qaq-p/1302944#M408883

logs show:

Feb 20, 2020 2:55:36 PM SEVERE org.jenkinsci.plugins.JiraTestResultReporter.JiraUtils logError
The issue might be deleted, or there is no internet connection, etc.
org.codehaus.jettison.json.JSONException: JSONObject["name"] not found.
at org.codehaus.jettison.json.JSONObject.get(JSONObject.java:360)
at org.codehaus.jettison.json.JSONObject.getString(JSONObject.java:487)
at com.atlassian.jira.rest.client.internal.json.JsonParseUtil.parseBasicUser(JsonParseUtil.java:192)
at com.atlassian.jira.rest.client.internal.json.CommentJsonParser.parse(CommentJsonParser.java:37)
at com.atlassian.jira.rest.client.internal.json.CommentJsonParser.parse(CommentJsonParser.java:27)
at com.atlassian.jira.rest.client.internal.json.JsonWeakParserForJsonObject.parse(JsonWeakParserForJsonObject.java:39)
at com.atlassian.jira.rest.client.internal.json.IssueJsonParser.parseArray(IssueJsonParser.java:146)
at com.atlassian.jira.rest.client.internal.json.IssueJsonParser.parse(IssueJsonParser.java:216)
at com.atlassian.jira.rest.client.internal.json.IssueJsonParser.parse(IssueJsonParser.java:88)
at com.atlassian.jira.rest.client.internal.async.AbstractAsynchronousRestClient$1.handle(AbstractAsynchronousRestClient.java:147)
at com.atlassian.jira.rest.client.internal.async.AbstractAsynchronousRestClient$3.apply(AbstractAsynchronousRestClient.java:189)