Steps to reproduce.
Install JIRA downloaded from www.atlassian.com
When setting up the instance, select "I'll set it up myself"
Navigate to UPM's "Find New Add-ons" page
Install an addon
An error occurred during installation. Refer to the logs for more information.
Plug-in is installed.
Note that problem is resolved after a JIRA restart.
This will be fixed with a JIRA 7.1.1 release. No UPM updates are needed to resolve the issue.
The fix is now available in JIRA 7.1.1.