JeanPierre the ManageWp support told me:
Hello Marcos.
This is Stefan from ManageWP, thank you for reaching back to us.
Regarding your issue, I've checked our system logs and I was able to find that all the restore processes failed with the same 'Internal server error 500'.
The problem is that the website is currently down because of the Critical error, and if you try to restore it by using the usual restoring method, the process will fail because the website is currently not operable, thus, the Worker plugin is unable to perform the request.
However, this is why we have an additional restoring method called 'Restore over an FTP' which allows our system to transfer your backup archive by using an FTP connection rather than the Worker plugin. In this case, our system will disregard the website state and move the backup directly to your server by using the FTP transfer protocol.
Still, by looking at your website the error when the website is loaded is "Error establishing a database connection' which doesn't necessarily mean that the website is down, rather the database credentials are not properly set in your wp-config.php file.
I would first kindly suggest you try to resolve this issue by double-checking the database credentials, here is how you can achieve that:
https://www.wpbeginner.com/wp-tutorials/how-to-fix-the-error-establishing-a-database-connection-in-wordpress/
After the website re-establishes the connection with its respective database, and the website is still down due to some WordPress-related error, please use this method to restore your website:
https://managewp.com/guide/backup/recover-my-website-with-ftp
If you have only SFTP credentials, please change the connection protocol to SFTP prior to entering the credentials into the FTP additional settings field.
Hope this was helpful, and if you still need assistance, please do not hesitate to reach out.
Kind regards,
Now my mission is to restore the connection with the database. Let's how it goes, a full day off is very sad for any site owner