Interesting error code.
X-cart recommends PHP version 7.2. Then, use code in X-Payments 3.1.4 that was depreciated in PHP 7.1.
Based on this error, the problem seems to be that the code doesn't support PHP 7.2.
Error: Uncaught Error: Call to undefined function mcrypt_module_open() in lib/XPay/Crypt/Cypher/Rijndael256.php:164
Isn't it true that Mcrypt was deprecated in PHP 7.1, and removed entirely from 7.2? Given that X-Payments 3.1.4 seems to either be using code deprecated in 7.1, or not properly removed within this update from 3.1.3, someone at X-Cart needs to get to work solving this problem.
Interestingly, when I change the site to PHP 7.0 all I get is a 500 error and the login page will not load at all.
Looking at the new files I extracted onto my system (If I were to install a fresh 3.1.4 version), there isn't any "Rijndael256" file, unless it is something created by the install, or that this is a leftover that was not properly removed during various updates. When I use FTP to look at what is on the server, I see that file and that it was modified today.
The PHP error from the log files show this error in previous days also. However, X-Payments was working properly prior to the update to 3.1.4.