if you say, you switched the theme there and back and the issue is gone, i suggest to check your Itemid (menu associations), it's suggested that VM's cart + both OPC's views are assigned to at least to a hidden menu and you set an OPC's checkout/thank you page Itemid withi it's config (it's per language setting)
Also we noticed recently:
On some themes which use Joomla Art's T3 framework (reviver or all joomla's art themes) we strongly suggest that joomla's art's plugin is called before "opc system (System - RuposTel One Page Checkout Master Plugin (OPC)) " plugin. I cannot suggest an exact name of the T3 plugin (try T3 framework, or ja-? plugins) as they change from theme to theme.
it's a known issue with T3 framework which overrides many Joomla's core files (could also be a security risk if they are not updated often) that if any other plugin intializes some of the core classes before the T3 plugin, the T3 plugin incorrectly tries to re-include the same class name with a different file path causing a blank page.
To find out all of your hidden issues or to get the actuall error, we suggest to try or use our fatal catcher plugin from here:
http://www.rupostel.com/plg_system_fata ... 5_or_3.zipbest regards, stan