At first I thought it may have been an issue with just one user but now several have come forward including a moderator.
Is it possible that you changed the Forum Security Key
This is the unique key which is used for securing your forum.
Control Panel > Primary > Forum Security Key
If it is changed, then all user passwords will be invalid, and will require each user to reset their password.
The field must be populated. If left blank, a new key will be generated.