Sometimes Joomla 4 shows there is an update for the "Installer - Install from Web" extension, but when you try to update it you end up getting a "PLG_INSTALLER_WEBINSTALLER_ERROR_PLUGIN_INCLUDED_IN_CORE" error.

This video shows a fix for this issue.

 

 

The full text of the error message that you get on the screen Extension Update:

Custom install routine failure. Before updating ensure that the update is compatible with your Joomla! installation. You are strongly advised to make a backup of your site's files and database before you start updating. × danger Error updating plugin.