Symptoms

When a user accesses the Universal Plugin Manager (UPM), the page is not accessible, even though all other pages are accessible:

Launching the browser developer tools (F12) will display the following error in the console:

ERR_CONTENT_DECODING_FAILED

Cause

The instance is running against a MariaDB database, which is not a supported database system. 

Resolution

1. Generate a complete XML backup of Confluence. This will be used to import Confluence into a fresh instance with a supported database. A database dump cannot be used in this case, since MariaDB is in use.
2. Restore the XML backup on a clean install of Confluence with a supported database (e.g., MySQL, PostgreSQL, Oracle).
3. Vote on the following feature request to support MariaDB in Confluence:  CONF-29060 - Getting issue details... STATUS