Password reset email is not valid? In setting up a new Drupal 7 site, I went to try out the password reset option, and while I received the password reset email just fine, clicking it takes me to a page with an error "You have tried to use a one-time login link that has either been used or is no longer valid".

I've tried sending the reset email a few times and clicking the link immediately each time, and keep getting the same error message. What sort of things make a reset link "no longer valid"? I'm working on a staging server that will be translated to a live server, so URLs and cookie paths might be getting tangled; where do I need to start looking to unravel this?

Note: The user accounts being tested were imported using the Migrate module from another system, hence their passwords are invalid, and I'm hoping users can just request a new password to login.
It's not documented nicely anywhere I can find, but it seems that you can't use the password reset option via email for a user who's an Administrator of the site. Instead you have to use some alternate method to change the database directly, like this.