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 Why add-on shows that license is empty when I provided valid license at plugin management page?

Probably there is a problem with UPM and Plugin License Storage plugin bundled with your JIRA. You can check this in logs looking for PluginLicenseStoragePluginUnresolvedException. Any users using JIRA 5.0.1+ could run into this problem because these versions of JIRA come bundled with UPM 2. Update the UPM in JIRA and re-install add-on fix this problem