Problem with update - UPS XML fails

I am trying to update a site from 1.3.9h to 1.5.4 and I am stuck with this plug-in. The entire site works correctly until I enable the plug-in, then accessing the checkout gives me a hard failure (500 error from server).

This error happens both on the full update site and a test site on the same server that has only a base install plus the plug-in.

Server Info:
Server Information

Server Host: zen.belmetric.com (50.97.151.126) Database Host: localhost (127.0.0.1)
Server OS: Linux 2.6.32-573.7.1.el6.x86_64 Database: MySQL 5.5.47
Server Date: 02/29/2016 17:05:19 Database Date: 02/29/2016 17:05:19
Server Up Time: 17:05:19 up 46 days, 2:22, 0 users, load average: 0.00, 0.00, 0.00 HTTP Server: Apache/2.2.15 (CentOS)
PHP Version: 5.6.18 (Zend: 2.6.0) PHP Memory Limit: 128M PHP Safe Mode: Off
PHP File Uploads: On Max Size: 2M POST Max Size: 20M
Database Data Size: 47,358 kB Database Index Size: 13,858 kB

Site is at https://zen.belmetric.com/

There are no logged errors either in the Zen Cart logs or system logs. Any ideas would be very welcome.

The UPS XML module is only posted to support ZenCart v1.5.1 and that version is only compatible with PHP 5.2.14 thru PHP 5.3.x

You posted that you are using ZenCart v1.5.4 and php v5.6.18

there is another UPS module for ZenCart v1.5.4 not sure if xml support is included
https://www.zen-cart.com/downloads.php?do=file&id=1293

Thanks for your reply. We've actually had that module working on other sites up to Zen 1.5.4 and PHP 5.5. The frustrating thing is there is no errors in server logs or MY_DEBUG logs. UPSXML log shows a successful Time in Transit request, then nothing. I've tried overriding error reporting in local and no change. Turnoff UPS XML and everything works.

Does anyone know another way of getting error messages to display?