...
The most possible cause of this issue is that your server may be using PHP extension "Suhosin" which is an advanced protection system for PHP installations. This extension has some restrictions that make returned URL from phpFox Store not be parsed properly.
If so, you should contact your hosting provider to update "suhosin.get.max_value_length" param to a higher value (at least 1000). This param has the default value of 512.
...