I think the browser update was a red herring.
If you get this error every time the code has run then you should ensure all design elements are signed with the same ID. Clean (Project -> Clean) the database if errors continue. I have found that sometimes 2 cleans are needed to resolve this (and other strange xPage issues).
However, I found that even after this has been done a user can still get this error randomly. When the user runs the code a second time it usually runs without error.
I would appreciate anyone's insight into why this could occur randomly.