Custom URLs aren't working in IE9 (not sure about earlier versions).
All other browsers are fine, but in IE our share buttons only share the page that is showing in the browser rather than the custom URL we have specified.
Could you confirm that this is a bug you're aware of?
(I'm fairly sure it's linked to the current Google +1 counter problem that you're aware of(see http://support.addthis.com/customer/portal/questions/337013-google-button))
Thanks!