A handful of people seem to have this issue. I think I've finally figured it out.
Who is you host provider? I ask because some hosts like fatcow.com do not setup the save path for the $_SESSION variable. So plug-ins that rely on it fail. For example WP_Surveys and eCommerce since they also both use this super global variable.
So download and open up your php.ini file and find the [Session] section and add this line:
session.save_path = /tmp ;
You might already have the text that says session.save_path = make sure you have /tmp after the equals sign. Don's forget that the line needs to end with a semi-colon. If that still doesn't work it could mean the tmp path doesn't exist on you server. You have to ask your host for the correct path. Also if you do not have access to your php.ini file as some hosts do not allow it, you'll have to request that they make this change.
Please let me and others in this thread know how you make out.