We recently upgrade from 2.12 to 3.2.0 MP4 - and are also having problems with many external users being able to log in.
It appears that the system is treating email addresses as case sensitive - and since the link no longer populates the UserName field they try to log in say in lower case and are denied access.
I did receive a script from Support to run that updates a couple of functions - the script is called boom_to_lower.sql - but no instructions on how to run it.
We'd like to see the product work by allowing the remote users to click on the link, enter their password and get their email - as they did in 2.12.
Thanks.