From using the automatic update package after using the smf converter at some point in the past:

Fatal error: Uncaught exception 'Symfony\Component\DependencyInjection\Exception\InvalidArgumentException' with message 'The service definition "auth.provider.smf" does not exist.' in C:\xampp\htdocs\pcwebplus\vendor\symfony\dependency-injection\Symfony\Component\DependencyInjection\ContainerBuilder.php:875 Stack trace:
#0 C:\xampp\htdocs\pcwebplus\vendor\symfony\dependency-injection\Symfony\Component\DependencyInjection\ContainerBuilder.php(483): Symfony\Component\DependencyInjection\ContainerBuilder->getDefinition('auth.provider.s...')
#1 C:\xampp\htdocs\pcwebplus\install\update\new\phpbb\session.php(579): Symfony\Component\DependencyInjection\ContainerBuilder->get('auth.provider.s...')
#2 C:\xampp\htdocs\pcwebplus\install\update\new\phpbb\session.php(500): phpbb\session->session_create()
#3 C:\xampp\htdocs\pcwebplus\install\install_update.php(120): phpbb\session->session_begin()
#4 C:\xampp\htdocs\pcwebplus\install\index.php(386): install_update->main('update', 'intro')
#5 C:\xampp\htdocs\pcwebplus\install\index.php(26 in C:\xampp\htdocs\pcwebplus\vendor\symfony\dependency-injection\Symfony\Component\DependencyInjection\ContainerBuilder.php on line 875

https://www.phpbb.com/community/viewtopic.php?f=466&t=2235881

I'm not sure what would be best to fix this, but the error is going to confuse and upset users.