Email Based Self Registration - Email Confirmation

We have enabled the email-self registration plugin in our moodle for students to setup their own accounts.

When they receive the email asking them to confirm their email address there is a problem with the link on the email - it is replacing the third character of their username with a equals sign. When the link is clicked the get the error "invalid confirmation data"

For example: http://moodle.example.com/login/confirm.php?data=FV8K2A9GRvGUT8f/joe=loggs

Does anyone know why this is or how to fix it?