J4.x:Upgrades to Joomla 4.0.2 fail with an AJAX error message
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.
Errors reported
Versions affected
General Information
This pertains only to Joomla! version(s): 4.0.1
What is the cause
A bug introduced in the updater of Joomla 4.0.1
How to fix
Please wait for 4.0.2; When you are already on 4.0.1 you will get an update of the Joomla Update Component that you have to install first and than you can go to 4.0.2