I have been digging in the forum because I’ve just had a problem explained here
Well, in my case, my app worked perfectly in development server, test server and production server. Today, suddenly, the error started to appear only in Live Server.
I have found what happen and I have solved it. And reason is quite simple: Not enough free space in HD. My server has a very limited HD space and perform a daily backup. When HD is full it seems MySQLd can’t perform any query.
Deleting some backups and freeing some space has solved the problem.
Hope it helps!