This error is similar in nature to the ”Supplied argument is not a valid MySQL-Link resource” error and are often caused by the same problem.

The SQL query noted in the error message is the very first query run by ExpressionEngine whenever it renders a page, so if there is a problem with the database then that is the query that will encounter the issue.  Typically, the error occurs when EE cannot communicate with the database, which could happen for a few reasons:

1. The database is down on the server

2. PHP is not configured to communicate with the database even though both are installed

3. The server has run out of connections to the database

The last issue can sometimes be corrected by switching from “persistent” to “non-persistent” connections, which is outlined in the Knowledge-Blog entry linked above.