Invalid translation language (de) specified. in Drupal\Core\Entity\ContentEntityBase->addTranslation()

Problem/Motivation

Adding a new translation apart from the default one results in an ajax error

InvalidArgumentException: Invalid translation language (de) specified. in Drupal\Core\Entity\ContentEntityBase->addTranslation() (line 985 of /var/www/html/releases/1/web/core/lib/Drupal/Core/Entity/ContentEntityBase.php).

In the past, this error was "only" in the log but not in the UI. After installing the gin theme the ajax issues are also visible to the UI.

After removing the Real time SEO field from the form display, the error is gone.

The problem seems to be that the route "pl/node/507/translations/add/en/pl" (for a new pl translation for example) is called synchronously by saving the new translation and after that the ajax call of this module takes place calling the same route again and therefor the exception is thrown because a translation for this language does already exist at this time.

Does anyone have an idea?