TYPO3 11 Upgrade: Backend 404 not found

Update nginx configuration

TYPO3 introduced new URL’s for the backend. If you’re using an nginx, please add the following to your nginx configuration:

 # TYPO3 Backend URLs
 location = /typo3 {
     rewrite ^ /typo3/;
 }

 location /typo3/ {
     absolute_redirect off;
     try_files $uri /typo3/index.php$is_args$args;
 }

Please make also sure, that no other nginx rule matches /typo3/

# remove typo3/ from other rules
location ~* ^/(typo3/|fileadmin/|typo3conf/|typo3temp/|uploads/) {
}

# so it's like
location ~* ^/(fileadmin/|typo3conf/|typo3temp/|uploads/) {
}

After updating your configuration restart your nginx or reload your configuration.

You can find a full configuration example here:
https://docs.typo3.org/m/typo3/guide-installation/master/en-us/In-depth/SystemRequirements/Index.html#nginx

Update Apache config

If you’re using the Apache webserver, you have to update the .htaccess file.

This change is documented here: https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/11.0/Breaking-93048-BackendURLRewrites.html