I contacted CCBill about this and in fact the Encryption Key was missing. I know it was there at one point but CCBill tech support had to re-enable Dynamic Pricing and provided me with a new Triple DES Encryption Key.
I tried another test order after completing this task and noticed it did NOT resolve the issue. The new error log looks a little different than the one you posted above. This one says:
LOG ENTRY: Wed Aug 7th, 2013 @ precisely 12:25 pm UTC
PHP v5.4.9 :: WordPress® v3.5.1 :: s2Member® v130617 :: s2Member® Pro v130617
Memory 5.49 MB :: Real Memory 5.75 MB :: Peak Memory 5.60 MB :: Real Peak Memory 5.75 MB
.com/?s2member_pro_ccbill_notify=1
User-Agent:
array (
‘s2member_log’ =>
array (
0 => ‘Unable to verify POST vars. This is most likely related to an invalid ccBill® configuration. Please check: s2Member -› ccBill® Options.’,
1 => ‘If you\’re absolutely SURE that your ccBill® configuration is valid, you may want to run some tests on your server, just to be sure $_POST variables are populated, and that your server is able to connect to ccBill® over an HTTPS connection.’,
2 => ‘s2Member uses the WP_Http class for remote connections; which will try to use cURL first, and then fall back on the FOPEN method when cURL is not available. On a Windows® server, you may have to disable your cURL extension. Instead, set allow_url_fopen = yes in your php.ini file. The cURL extension (usually) does NOT support SSL connections on a Windows® server.’,
3 => ‘array (
(user data erased).