It looks like you are using the free license for Splunk since I don't see your login name in the top of the Splunk window. Splunk's free license limits some features and it's possible that the app is trying to use those features and throwing the error when it can't access them.
I didn't see anything in the app's release notes about limitations when using the free license. You might want to email firstname.lastname@example.org and ask them if they know why their app isn't working on your server. It looks like a bug with the add-on to me.