Drupal - Bootstrap failing with Drush 9.1.0

I has this issue and found this to work:

rm -Rf vendor/drush
composer install

Credit: https://github.com/drush-ops/drush/issues/2959#issuecomment-332054773

This deletes drush and uses the composer.lock file to re-instate drush from the version specified there.

Version of drush is 9.5.2 on my setup (after running above command at time of writing).