? character added to login

I have used this plugin on many sites (thank you!) without problems.

A problem started on this development site a couple of days ago, and I started having trouble logging in. After much messing about I found out that when this plugin is activated it is adding a ? between the final / and the changed part. This disappears when I deactivate WPS hide login, ie it is not there when logging in to wp-admin.

I have been going round in circles trying to find out what is adding this ?. I have deactivated all plugins except this one and it is still there. It does not happen on any other site.

The page I need help with: [log in to see the link]