The generated link does not comply with the web standard:
Simply encode the ampersand (& -> &) and everybody will be happy. Adblock will understand it and the validator will give the all-clear.<a href="abp:subscribe?location=http%3A%2F%2Fwww.example.com&title=example">Subscribe</a>
Additional info:
http://www.htmlhelp.com/tools/validator ... s.html#amp
EDIT: subject altered.