Yeah, that's come up a number of times here and it's related to the HTTP Referer Check (Control Panel » Primary Settings » Advanced Options)... you can search here for it.
Users who get that error should be sure your domain is added to their firewall/security suite whitelist. The referer check makes sure people are posting thru the forms on your site rather than remotely, so it's advisable to keep it enabled. But turning it off will stop the errors...
