After posting this puzzling question, it finally occurred to me go through the basic troubleshooting procedure of deactivating, then reactivating all plugins, one by one. When Role Scoper was deactivated, the error went away.
Basically, Role Scoper works fine when you get the settings right. I am using it together with WC Members to make a members site. There is more detail here on how Role Scoper made it possible to give users special posting and viewing capabilities, and have an entry level of users with the Contributor role who have the same capabilities, but whose posts are moderated.
The Role Scoper and WC Members plugins have a few important things in common. Their developers provide uncommonly thorough documentation, they release upgrades regularly and they offer excellent support through their forums.