Password Reset broken

Hi,
I have found that the password reset function is not working properly on my site since version 4.7.5 of the plugin (i.e., it breaks if I change to version 4.7.6 and subsequent versions, but works if I rollback to 4.7.5 or earlier). When a user clicks on the ‘reset password’ link in their email, it sends them to the login page rather than a reset page and the url is: [website root]/pm_profile/pm_login/?login=invalidkey
Any ideas what’s happened here?
Thanks, Paul