I have two sites with ENFOLD template Error establishing a database connection

I have two different sites on the same hosting account with the same IP address, both with the ENFOLD template.

One very old site – https://www.kav-acher.com/
Recently I had some problems with it and had to restore it from a backup I found (not the perfect backup – but better than nothing) I used the All-in-One WP Migration plugin.

The second website is completely new – https://www.alinacademy.co.il/
I had a design on it that I started to build, because the actions were a little limited, every time I switched to the mode – the layout editor ‘advanced’ this error appeared.
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Error establishing a database connection
This either means that the username and password information in your wp-config.php file is incorrect or that contact with the database server at localhost could not be established. This could mean your host’s database server is down.

Are you sure you have the correct username and password?
Are you sure you have typed the correct hostname?
Are you sure the database server is running?
If you are unsure what these terms mean you should probably contact your host. If you still need help you can always visit the WordPress support forums.
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So I deleted the site and the existing database and tried to install everything from scratch, but the error still appeared.

After they connected the domain of the new site to me and I installed WordPress and the template, I saw more and more errors like this and now on both sites.

I consulted with the storage company and we did not find a solution for this.
Is it related to the template or rights registration?