I don't experience the error when I visit the problem on our server. The error sounds like it may stem from some bug that was possibly patched between the version of WeBWorK you are running (2.9) and the one we are running here (2.11).
If you are able to upgrade your server, you might try upgrading to 2.10. (I don't know if 2.11 is really ready yet---Geoff and others may still be doing testing. But if you are adventurous, you could try 2.11 as well.)