We've become aware of an issue that is making it impossible to login via the front-end due to Kontera's ads delivery system. Please disable your Kontera ads for the time being until they provide an update or a fix.
Update: The developers of Kontera are actively working on fixing the issue.
Update Nov 18th 09: Resolved.
Update April 22 10: Some customers have started reporting issues again.
Update April 26 10: It seems that the issues should now be resolved. If you're still having issues please get in touch with the people @ Kontera.com