I had this same problem. I looked at the /paypal_wpp/includes/config_ec.inc.php and noticed that the path to the certificate is
$paypal['certificate_file']=DIR_FS_CATALOG . MODULE_PAYMENT_PAYPAL_EC_CERT_FILE;
I checked my /includes/configure.php and found that I had DIR_FS_CATALOG set without a trailing slash.
That means the Paypal Pro routine was looking for my certificate file at "/var/www/sitename.compaypal_wpp/certs/cert_key_pem.txt"
I went to the configuration of the Paypal Website Payments Pro in my store's Admin and I added a slash to the beginning of the path: /paypal_wpp/certs/cert_key_pem.txt.
After doing that it found the certificate and the error went away. It works with and without an SSL certificate.
I noticed that above in your post that you have your certificate set to catalog/paypal_wpp. You should be able to just drop the catalog but leave the /paypal_wpp.
Nice contrib!!
Thanks