Yeah, I’m thinking on how not to destructively test that. This theme has worked through several updates without a problem.
I removed all plugins from the folder as well as disabling all of them. I hadn’t thought of changing the theme. I am using a Pagelines commercial theme. I am reticent about changing that, because because I’ll lose widget data (or at least that seems to be what happens when I switch themes that don’t have common widgets).
If the theme is the cause, I still need to know how to diagnose that.