Another note. Actually I received four log files after two test orders. Two are above, regarding the sizeof() warning. Another two are Error while sending QUERY packet. See below:

#1 mysqli_query() called at [/public_html/test/includes/classes/db/mysql/query_factory.php:44]
#2 queryFactory->query() called at [/public_html/test/includes/classes/db/mysql/query_factory.php:260]
#3 queryFactory->Execute() called at [/public_html/test/includes/functions/sessions.php:77]
#4 _sess_write()
#5 session_write_close() called at [/public_html/test/includes/functions/functions_general.php:41]
#6 zen_redirect() called at [/public_html/test/includes/modules/pages/contact_us/header_php.php:98]
#7 require(/public_html/test/includes/modules/pages/contact_us/header_php.php) called at [/public_html/test/index.php:35]
--> PHP Warning: Error while sending QUERY packet. PID=879489 in /public_html/test/includes/classes/db/mysql/query_factory.php on line 44.

I did some research and it looks like this error is on server side, not ZC side. I'm not sure if it is related to issue above.