Not sure if it is related to the checkout issue, but you can add this code:
Code:
$p = preg_quote($p, '/');
right before this one:
Code:
if(preg_match('/' . $p . " ([^;]+)/Ss", $preg[2],$ppreg)) {
in the "include/adaptives.php" script, to fix the error reported in logs.
Note: use FTP/SSH access to alter the PHP scripts.