Lately I was updating some Drupal websites and suddenly CKEditor was broken. It seems like it now needs its libraries from separate libraries folder instead of shipped with the module. Here is how I fixed it. I added the following packages to composer.json in the repositories section:
composer require drupal/core-composer-scaffold composer require drupal-ckeditor-libraries-group/link composer require drupal-ckeditor-libraries-group/fakeobjects
This will download the plugins f.e. to web/libraries/fakeobjects, and this is where from Drupal 9.3 CKEditor will seek them.
Concerning fakeobjects it is possible you need the drupal/fakeobjects module as well.
Clear caches and your CKEditor is back!
Still no success? Look in your console, maybe you had some extra plugins as well. look them up and add them as a package as well.