Hi,
Great ! I think we're moving forward.
What's great is the response.txt file you provided.
In it, you can find the abridged PHP error message:
0 - implode(): Argument #2 ($array) must be of type ?array, string given
Ideally, it would be great to replicate the same issue with the "debug" setting of the Joomla configuration turned on. That response.txt would then contain the full error message with the file path and line number where the problem is.
Without it, I can only guess what the exact problem is. Potentially, it could be in anything linked to HikaShop, not necessarily in the payment plugin.
However, supposing that the error is inside the code of the Worldpay plugin, there are only 2 "implode" calls in there.
One of them is this one:
$hostError = (!preg_match('#('.implode('|',$ips).')#',$ip) );
it checks the IP address of the worldPay server based on the IP addresses configured in the settings of the WorldPay payment method. That setting is empty by default. If you filled it, it could well explain the problem.
So supposing this setting is filled on your website, try changing this line in the file plugins/hikashoppayment/bf_rbsbusinessgateway/bf_rbsbusinessgateway.php to:
$hostError = (!preg_match('#('.$ips.')#',$ip) );
and it should fix the problem.
If that's not the problem, then will need the response.txt with the debug turned on to be able to tell you what to do to fix the problem.