Okay, nothing had changed in TFS (at least to my knowledge and I'm the administrator!).
I received the error "TF400898: An Internal Error Occurred", when clicking on a previous sprint in the backlog. A colleague also received the same error when trying to move an item within an iteration of the current backlog.
Navigating to each team, opening the 'Configure work areas...' menu and doing nothing more, appeared to correct the error (or was it a coincidence?).
Has anyone got a clear and definitive answer as to why the error occurred? More importantly, how to correct it? Surely there's more to my simply browsing between the teams.