Troubleshooting:
- If you have little content on your Drupal 7 or Drupal 8 website, a fresh install of Drupal 9 is typically much more efficient than going trough the upgrade hassle, which is only announced to be easy, but often accompanied by lots of problems. This is especially valid if you are doing a theme customization with a new theme.
- If "Drupal console prevents update to Drupal 9 via composer" use this solution
- Switch to PHP 7.4, PHP 7.3 and 7.2 often cause "The website encountered an unexpected error" and error messages when clearing cache and updating db with drush
This problem really sucks and appears also with other modules: You have requested a non-existent service "entity.manager"
Migration:
- Use the module Upgrade status to prepare a migration from Drupal 8 to Drupal 9. Upgrade all modules to latest status also when shown under "Drupal 9 compatible"
- Minimum Drush version is 10
- Use an up-to-date version of composer
- Try the first 9.0.0, the latest version and maybe also intermediate versions if you encounter problems
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