Cindy,
I did a full upgrade from 2.2.0 (not working) to 2.2.1 following every step in the installation pages including merging files etc (I overwrote files, I did not uninstall - interestingly the same version 2.2.0 is stated in admin/configuration/opc) .... and the same issue is happening - so my logic tells me one of two things; I have still made an error in installing or more likely that this has little to do with OPC and more to do with the ZC payment system not responding correctly due to a corrupt or missing file or a file in OPC which is interfering in the process. When OPC is disabled I get the same result.

A real head shaker - on another site that I upgraded from 2.0.5 to 2.2.0 I could not get Guest checkout to work so I disabled it and enabled the PayPal Express button and customers can now checkout directly via PayPal and/or via login. On this particular site neither happens.

I checked every file associated with OPC on the site I can at least get working without OPC and compared them to the corresponding files on this problem site - they all matched. So I am well and truly stumped.

My host has no record of any 500 server errors and there are none in my public_html/logs folder

So before I revert to 2.0.3 as a fall back I need to try to understand why these url's that return a blank page are not proceeding somewhere, anywhere
https://www.treeoflifejewellery.com....eckout_payment
and
https://www.treeoflifejewellery.com....er.php?type=ec

What files being called are not being actioned? obviously something is saying I won't proceed - a file in OPC not talking to the standard checkout system maybe?- which files are shared / common if any?

Anything you can think of that I can investigate would be wonderful.

cheers,
Mike