“invalid key” when resetting passwrd!

My users are unable to reset their passwords since everytime they click on the reset link in the email, they get the message that the key is invalid.

I can’t manually reset the passwrd in phpMyadmin each time a user forget their passwrd. I have more than 100 users so that is not an option.

Anyone know what’s wrong with the reset function? Or what I should look for?