Ben,
You might try deleting related cookies, logging in via another browser, resetting password, changing the parameter value that Hazi Gharagozlou suggests in this[1] thread, or something completely different: finding another browser window where you might be able to refresh an earlier admin session. This last method saved me a couple of times.
Torben
1. https://openacs.org/forums/message-view?message_id=167212