Ciao,

probably durning FTP upload process not all file(s) were uploaded, something is missing/or too much.

1. Please download Joomla 4.1.5 update package from joomla.org

2. Unzip

3. Upload/replace all files from folder: /administrator it may help.

if will not help upload/replace all files also from those folders: plugins, includes, layouts and media.

I followed your instructions but it doesn't work. Same error.

In that case please:

1. Delete this file (if you have it): /libraries/autoload_psr4.php or similar with "autoload...." name.
2. If (1) will not help - delete whole folder /libraries/
3. and upload that folder from fresh Joomla 4.1 package.
4. Then if you will be able to login to admin area update Joomla 4

Tip based on: https://issues.joomla.org/tracker/joomla-cms/20828

If 1..3 will not help, you have to install J3 Quickstart and upgrade to J4 from it. I don't have alternative package to share :(

i had to install j3 and upgraded to j4.

thanks

To be honest even in Google were only 3-4 cases when somebody in the past had this issue, so it's very very rare.

Hopefully, J3-J4 upgrade method helped.

j3-j4 upgrad worked. thanks