I know this is an old thread but I ran across the same issue and stumbled on this thread which actually lead me to checking the log files...
Anyway, I was getting the following in my cache/myDEBUG-adm logs :
As I'm on shared hosting, I was able to fix by adding an .htaccess file to the root of the zen cart installation containing the following line:
PHP Fatal error: Allowed memory size of 8388608 bytes exhausted (tried to allocate 311296 bytes) in *****/zen-cart/includes/modules/payment/paypalwpp.php on line 1355
You could also address this by modifying your php.ini configuration.
php_value memory_limit 64M