I don't disagree, but both you and I know that the name (and folder) for the local settings file is complete arbitrary as long the file actually is included from the used settings.php file ;-)

I support the DDEV suggestions, but you could try to understand what is going on by adding the -vvv option.

Maybe, but I thought the difference in commands only appear if you try to update Drupal core separately? This questions is about updating everything in one go. I don't have time to test. If you verify that a different command is needed, feel free to edit.

You are correct. Sorry about the non-helpful comment. This is coming from Drush trying to backup your files before upgrading (or something). Drush doesn't use file_private_path - it uses it's own setting. See for example: stackoverflow.com/questions/26879225/…

Then I'll remove my downvote (if I'm allowed to).

NB! The caret notation "^9.1.5" doesn't lock the version - read getcomposer.org/doc/articles/versions.md - it just blocks upgrading to Drupal 10.

The drupal.org documentation - drupal.org/docs/updating-drupal/… - says to use composer update drupal/core "drupal/core-*" --with-all-dependencies. Does it help?

Just for the record, since this question suddenly was listed as active again: Drush hasn't supported installation through Pear for many years. Read drush.org/latest/install for current install instructions which requires the use of Composer.

I assume that message is coming from the "Backup and migrate" module. If you disable it, are you able to update with Drush? If I'm right about B&M, I think you should check the module issue queue. You could update your question with version info for B&M.

Is the problem resolved? Since you have Drupal 8.4+, you should use Drush 9 - ref chart leymannx linked to.