This issue appears to be either some discrepancy under the Splunk app configuration in Jenkins or a version issue.
Have you tried to insert the HTTP Input Host without the fully qualified domain?
What is the Jenkins version you are currently using? You may need to upgrade your Jenkins version or use a different version of SplunkApp in Jenkins( May need to use an older version )
as we are exporting data to splunkcloud the host name is something like: http-inputs-xx.splunkcloud.com so the HTTP Input Host without the fully qualified domain will not work for us...
On some masters we have the latest version of both Jenkins and Splunk plugins.
I see. Have you check the HTTP event collector settings?
It may need to be enabled or maybe the metadata ports may need to be white-listed.
Unfortunately that is all I can think of right now.