Such errors may happen if you have “virtual” themes instead of “normal”. This sometimes happens during the upgrade. The issue described in detail under Let’s fix the "Required parameter 'theme_dir' was not passed" error article.
Here a list of steps on how to fix the error.
- Go to PhpMyAdmin > Your Magento2 database.
- Find the "theme" table.
- Make sure that all theme types are set to '0'. In case any of them have type = '1' change it to '0'.
That's it. Please check. We think the error has already been fixed.
