TimeCapsule won't let me type password for "delete all backups of..."

I've been using this TimeCapsule/TimeMachine for a couple years. I'm trying to open up some space. So I just excluded a couple folders from being backed up. Now I'd like to dump the old backups of those folders. So I go into TimeMachine, go to latest backup (not "Now/Today"), click on folder, hit "Delete all backups of..." from the action/gear menu, and it warns me that it can't be undone. Then it prompts me for admin name/password, but it won't let my type in the password!

It will let me hit the OK button (at which point it says it's a bad password), or hit the Cancel button to cancel, but it won't let me type anything. (Not into the Name field either, though that doesn't matter since it's prefilled with the correct name.)