How to discover disconnection with database

Several times a day, I have been facing the following error: “Error establishing a database connection”. I have 4 sites on this server (with EasyEngine), however they are all institutional, and the accesses are very few. In WordPress installations, there is nothing strange, everything is updated to the latest versions, the plugins used are all reliable and there are no peaks of access.

I know this error is quite generic, so how can I identify the cause of this error, and fix it?