J - My apologies for the delay in responding and thanking you for your help. I was distracted nailing down a number of server issues including the problem I inquired about.
It appears the problem I originally described was caused by a missing PHP module.
For other similarly server-illiterate folks with similarly mysterious obstacles and error messages cropping up in Omeka, I’ll pass along my hard-earned advice to check with your provider to make sure ALL of the Omeka-required modules are not only available on but actually INSTALLED and enabled on your server. My host, AISO.net, were VERY helpful and diagnosed and tweaked settings themselves. And they’re solar-powered!