The default key works on all sites, regardless of URL. I do however encourage all users to get their own key.
I think I know what's going on. No default module has been set.
Go to ACP -> Applications, click on the "Member Map" row to make it expand the module(s), then click the star next to the row that should appear to make it the default one.
If that doesn't work, I may need ACP, and maybe FTP, access to figure this out for you.