All in one causing connections to be lost

The installation of the plugin causes my admin website connections to be lost and the next alert to be displayed specially when trying to edit products or using other plugins like invoices:

Connection lost. Saving has been disabled until you’re reconnected. We’re backing up this post in your browser, just in case.

Then:
Error establishing a database connection
This either means that the username and password information in your wp-config.php file is incorrect or we can’t contact the database server at resteltravel.netfirmsmysql.com. This could mean your host’s database server is down.

Are you sure you have the correct username and password?
Are you sure that you have typed the correct hostname?
Are you sure that the database server is running?

https://www.optiweb.ca

The page I need help with: [log in to see the link]