Problem

Navigating to Confluence Admin >> Manage Add-Ons results in Page Not Found. The URL shows the correct path: http://<ConfluenceURL>/plugins/servlet/upm

Cause

1. Either corrupted plugin caches, or the UPM fails to startup due to this: Plugins are disabled at startup and various timeout errors appear in the logs after startup
2. It is also possible that the issue is due to compatibility of Atlassian Universal Plugin Manager Plugin with the Confluence version

Resolution

For Cause 1

Shutdown Confluence, then remove the following from the Confluence Home Directory (refer to Confluence Home and other important directories for more information):

• bundled-plugins
• plugins-cache
• plugins-osgi-cache
• plugins-temp

After that, startup Confluence. If that does not solve the issue, please refer to this article: Plugins are disabled at startup and various timeout errors appear in the logs after startup (the UPM may not be starting up due to a timeout)

For Cause 2

Verify the version of the Atlassian Universal Plugin Manager Plugin and compare it against the below page where it shows the support versions on the second column:

• Atlassian Universal Plugin Manager - version

If you have incompatible version of the Atlassian Universal Plugin Manager Plugin with the Confluence, please follow the below steps:

Confluence stores add-ons such as UPM in its database, so the process for updating it by file replacement is a little different to other products.

To update the UPM file in Confluence:

1. Download the UPM JAR from the Atlassian Marketplace. 
2. Stop Confluence. 
3. Remove the existing UPM JAR file from <install-directory>/confluence/WEB-INF/atlassian-bundled-plugins. 
4. Copy the new UPM JAR file into the same directory. 
5. Clear the Confluence plugins cache. 

6. Downgrading only:
   

   Always back up your data before performing any modifications to the database. If possible, test any alter, insert, update, or delete SQL commands on a staging server first.

   DELETE FROM plugindata WHERE pluginkey LIKE 'com.atlassian.upm.atlassian-universal-plugin%';

7. Restart Confluence