I ran into this problem after an interruption during add-on update from the administration panel.
When I re-entered add-on manage page, all of the "update" button on the right side of all add-ons disappered. When I click a random add-on to expand the detail box and click "update" button from their, an error message poped up says
"An error was encountered while updating this add-on."
Every add-ons on my JIRA got the same error messages including FOLIO and JIRA Agile.