Any luck if you reload the extension, or removing from the Dashboard and adding it again?
You may have this link already but just FYI it has info on managing extensions in Tableau Server
Looks like it must have been an issue with 2018.2. Upgrading to 2018.2.1 resolved the issue. Based on experience, this is the version to use it seems.
Hi Rob, thanks for the update, glad to hear 2018.2.1 resolved this!