Disable all your other plugins except this one and carry out another test. If that does not work try to disable the plugin and enable it again.
Also check your .htaccess file and make sure there are no anomalies in the file. Speak to your host about this issue.
You could as a last resort delete the plugin and install it again. Create a backup just in case something goes wrong.
Do you have a cache plugin installed? If you do, is it set up correctly to allow your shopping cart to function properly.