I changed the custom admin URL via the admin.
I've set Stores -> Settings: Configuration -> Advanced: Admin -> Admin Base URL -> Use Custom Admin URL to Yes
I've set Stores -> Settings: Configuration -> Advanced: Admin -> Custom Admin URL -> Custom Admin URL to adminxyz
but just after saving it showed me the 404 site.
Now I can't access the admin (backend) anymore:
So I opened phpMyAdmin and noticed that in the table
core_config_data the entries with path web/unsecure/base_url and web/secure/base_url changed to adminxyz.
So I changed them back to my domain https://www.example.com/
I also set the of path admin/url/use_custom from 1 to 0.
But I still can't login in the backend.
I already flushed the cache and reindexed.
If I execute
php bin/magento info:adminuri then I get:
Admin URI: /adminxyz
I even made this entry in my app/etc/env.php:
<?php return [ 'backend' => [ 'frontName' => 'adminxyz' ],