Mismatched entity and/or field definitions

That's strange, it reads as if the entity types from ECA are not known on your production site. But if ECA is enabled correctly, then those entity types must be there. Don't see what we can do as module maintainers to resolve that. Something must be wrong with the installation. You should try refreshing the cache a couple of times, or rebuild the registry of the site, see the core/rebuild.php file in Drupal core's directory.

The strange point, for me is:

• The error warning is not shown in my DEV site
• The module is running OK in Production

I've tried: https://www.drupal.org/node/3034742#comment-14183957 →

without success.

By the way, I deployed ECA using the usual steps from DEV to Production:

git pull
composer install --no-dev
drush updb
drush config import
drush cc

It was an early version of ECA a year ago...

Not sure if I could just uninstall and reinstall again in Production ECA module... Could it do the trick?

Not sure if I could just uninstall and reinstall again in Production ECA module... Could it do the trick?

Sure, I'd always do a db backup first. Just in case. And/or you could copy your production DB to your local installation and see if it is still the same there. This would help with debugging.

I have this warning on several sites, and so far have ignored it without any apparent consequences...

Just re-checked all our installations and it's not there anywhere. Also, I've never seen that elsewhere before. So, I went through the code in Drupal core to find out, what's producing such messages. Found this:

      $original = $this->entityLastInstalledSchemaRepository->getLastInstalledDefinition($entity_type_id);

      // @todo Support non-storage-schema-changing definition updates too:
      //   https://www.drupal.org/node/2336895.
      if (!$original) {
        $change_list[$entity_type_id]['entity_type'] = static::DEFINITION_CREATED;
      }

So, Drupal core is not able to track changes of non-storage-schema-changing definition updates yet, see #2336895: Allow entity type and field storage definition objects to be compared for definition equality → . Nothing a module can do about it, need to wait and see if core eventually fixes that.