Did your upgrade process validate operation in a test environment before being put in service on a live site?
Was the module removed and reinstalled as part of the upgrade?

Personally not familiar with the payment module in question, but other than making changes to support the php version being used, there wasn't anything that significantly impacted payment modules in ZC 1.5.6/1.5.6a.

BTW, what PHP version is being used on the site?