Thanks d4z_c0nf and ElectricFeet for your support on this. ElectricFeet, I took your advice, deactivating all plugins and reactivating one by one to find the source of the problem. Indeed, it was Photon!
For what it’s worth, d4z_c0nf, yes, “Retina support” is enabled.
Thanks again. So very glad to know help is out there. And now I can go back to learning code and updating the site without those ugly Undefined Offset notices.
