From your description and without further investigation, it seems that the problem may be related to your host server (mostly certain php builds), since the ZC Application has not been recently modified on the server.

Please review recent history of php build implementation on the host server and see if the problem coincides with any such build implementation.

My suggestion is to install a debug logging utility to track php errors:
http://www.zen-cart.com/index.php?ma...roducts_id=860
You may find a case where an error is never reported (the cybs_sha1() function being a well known case).
The incompatible php build environment is not easy to identify and, I found it mostly a case-by-case process.

Updating to ZC 1.3.8 may be considered as a second step.

deBeaujeu