I tried to use this plugin with another database in the same server (localhost) and had the same problem as mikkelbreum. This happens because you are setting another connection with the same server and then closing it.

To solve this problem you have to modify in plugins/authentication/extdb.php, instead of mysql_connect you have to change it for mysql_pconnect. That worked for me.

Also I set in the response another parameters such as the gid and usertype to avoid permission problems.

It would be nice if the gacl permissions could be set from the external database as well. I don't know if this is possible yet but I'm working on that I'll let you know if I get something.