It is probably the old issue with "www." versus no "www." in the domain and being redirected back to the opposite one.
The best solution is to see if you get redirected back to the www or non-www version of the site after going to paypal and then update your .htaccess to make sure people are always on that version.
You can use this code to do it:
http://stackoverflow.com/questions/4916222/htaccess-how-to-force-www-in-a-generic-way