Hello,
Regarding the MIGS plugin source code, it is very unlikely that it can cancel the order without displaying a message in Joomla system.
The only solution would be that the call made with cURL do not return any content nor error.
So you should really check what you have entered for the "URL" setting of that plugin because it looks like that this URL is not working.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.