Joomla 4.0.2 へのアップグレード時に AJAX エラーメッセージが出て失敗する
From Joomla! Documentation
In the process of clearing up issues discovered with the first major release of Joomla 4.0.0 a patch was merged which has broken future updates from Joomla 4.0.1 to 4.0.2 Update packages.
This pertains only to Joomla! version(s): 4.0.1
A bug introduced in the updater of Joomla 4.0.1
- Please wait for 4.0.2
- When you are already on 4.0.1 you will (once published) get an update of the Joomla Update Component that you have to install first and than you can go to 4.0.2
- Updates from 3.10.x or 4.0.0 to 4.0.2 will not be affected.