Found the problem. I’ll post the solution; perhaps it will be helpful for others.
I’d deactivated plugins in order to setup a subdirectory network (for use with Group Blogs). In doing so, the bbPress installation became “disconnected.” I went to Network Admin, set up bbPress from an existing install (the bb-config.php file was extant), and voila, reconnected.
Resolved, with gratitude, due to the collective knowledge of a number of BP people who’d posted about related issues. Thanks!