User with assigned task unable to open task to complete it

I have a SP 2010 custom list with an approval workflow attached.

I have set up permissions for a number of users to have access to the site.

A workflow was started and the task assignment email went to the first user.

When he clicks on the outlook message's "Open this task" button, he gets a popup that says "No such interface supported"

This works fine for me. It worked for others that I had try it.

What might cause this particular error to appear for a user?