PARTIAL DIAGNOSIS:

Having reviewed threads on similar 500 errors on this forum, I troubleshot and revealed the causal file: auto-loaders

For the Sales-Report-Exporter Mod, I had needed to include a new auto-loader to include the said functions_lookups.php file for one of its functions. Deleting this auto-loader solved the problem.

The question then becomes, for me, why has this auto-loader worked until this upgrade? Were we allowed to redeclare functions until 1.3.9(h or otherwise) and only now will zencart complain about it?

I guess I'll need to basically figure out which functions_lookups.php functions we being called by the exporter mod and create a new extra_functions file specifically for it.

Thanks if anyone can shed some light for me.