by Davo Smith.
Well, I can't say for certain what is going wrong, but it looks like it is failing to find the right userid to view when you click on the second user to grade.
When it looks for the user in the database, it doesn't find them, then the error message you got happens due to the 'user' being 'false' (database record not found) rather than an object containing the user details.
I can't really help much more than that - but it does look like a core Moodle bug to me - make sure you are running the very latest code (Moodle 2.9.2) and, if that doesn't solve your problem, you should open a ticket on tracker.moodle.org to see if someone can investigate further.