PluginNotFoundException: the entity type does not exist, while adding configuration

Hi,
I'm trying to understand the reason why and how to fix the following issue: When trying to 'Add Fast Autocomplete configuration' (no other existing configuration yet), I'm getting the following message in dblog:

Drupal\Component\Plugin\Exception\PluginNotFoundException: The "xyz" entity type does not exist. in Drupal\Core\Entity\EntityTypeManager->getDefinition() (line 139 of ../core/lib/Drupal/Core/Entity/EntityTypeManager.php).

as well as a white page 'The website encountered an unexpected error. Please try again later.'

Any hint about how to solve this issue or where to look for, would be really appreciate. Thanks!