[Steven Zahm]

@ Chris

The Pie Register plugin is calling a core WordPress action, wp_login on login which Link hooks into but it is not passing both of the required parameters so when other plugin such as Link hook into it, it will fail with errors. That is a bug with Pie register.

More can be found out about this bug here:
http://connections-pro.com/faqs/pie-register/

Hope that helps!