I checked out the link you gave and it looks like the map loads fine, but then the server generates a
"500 Internal Server Error" when trying to load the placemarks, so the plugin kills the map and displays the error message.
Can you copy/paste the relevant lines from your Apache error log? You can ask your web host for help finding it if you don't know how. Also, ask them if they have the
mod_security module for Apache installed, as that can sometimes cause problems too.
It could also be that there's a problem with one of the individual markers, so you can try setting them all to drafts and then turning them on one-by-one in order to pinpoint which one is causing the problem.
I realized recently that the AJAX that's used to retrieve the placemarks isn't really necessary anyway, so I'm going to be taking it out in the next version, which should solve this problem. In the mean time, you could also try downgrading to version 1.1 until the next release comes out.