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Supplied argument is not a valid MySQL-Link resource
ExpressionEngine returns the following warning: "Supplied argument is not a valid MySQL-Link resource."
Troubleshooting
This error is usually returned in one of two cases:
The MySQL server is down
This can only be resolved by the database administrator who should be contacted immediately.
MySQL has run out of database connections
If the server is working in principle, MySQL might be running out of database connections. If that is the case then ensure that the database is using non-persistent connections. This can be set via Control Panel > Admin > System Preferences > Database Settings, or by adding the following line to config.php:
$conf['db_conntype'] = "0";