Delete the file administrator/cache/autoload_psr4.php

If that does not solve, then unzip the Updata package and again delete the file administrator/cache/autoload_psr4.php

Actually, see https://docs.joomla.org/J4.x:Fix_%22Reinstall_Joomla!_core_files%22_in_4.4.0_broke_Installation_and_backend_login

@woluweb
The update has been run without issues. The only nasty thing is the error message. After that, frontend and backend work without any issues.

So the message came only one time and after using the back to dashboard button all works without having to delete the autoloader file manually? If so, does the backend show the right version, or still the 4.3.4?

The class mentioned in the message belongs to the Backwards Compatibility plugin for J5, which is already installed on 4.4.0 but not doing anything, just for being present when later updating to to 5. No idea what could try to access this class in J4.4, except of the procedure which generates the autoloader file.

We might have to dig deeper to see what happens. Unfortunately I cannot reproduce the issue.

@richard67

So the message came only one time and after using the back to dashboard button all works without having to delete the autoloader file manually? If so, does the backend show the right version, or still the 4.3.4?

The message come just once. After navigating to the dashboard, it shows the proper version 4.4.0. I didn't delete the autoloader file.
If a remember correctly, I saw this message two times when updating a site to 4.4.0; did that for about a dozen sites.

Maybe some kind of caching issue? @dautrich Do you have the PHP opcache extension enabled on these 2 sites?

@richard67

I never cared about opcache. System information tells me that Zend Opcache is up and running:

All my websites are hosted by 1blu. So I assume that this cache is up and running for all my websites (I didn't check closer).

Yes, but it could be some kind of timing issue with that. Anyway that's just an idea and would be hard to reproduce.

The updater should invalidate the opcache for every updated and deleted file, so that should not be an issue, but who knows, maybe there is still something missing.

Maybe it would be better not to invest too much time into this issue.
Another idea: Why not add a sentence to the FAQ? Something like:
“Some users have occasionally seen the following error message: xxx. This shouldn't be seen as a serious error message and can be ignored.”

Another idea: Why not add a sentence to the FAQ? Something like: “Some users have occasionally seen the following error message: xxx. This shouldn't be seen as a serious error message and can be ignored.”

Godd idea. @dautrich Can you do that? Or maybe release managers @laoneo @MacJoom ?

@richard67 I don't have any rights on joomla.org

Didn't happen on later updates anymore. No reproducible. Closed by the author.