Is it possible that MySQL is configured to only listen on a specific IP address, and not localhost?

Yes. Check the `bind-address` setting in your MySQL configuration file (usually `my.cnf` or `my.ini`). If it's set to a specific IP address other than `127.0.0.1` or `0.0.0.0`, MySQL will only accept connections from that IP. Change it to `0.0.0.0` to listen on all interfaces, or `127.0.0.1` to listen only on localhost, then restart the MySQL server.