Thanks, @Frank_Lepore . I think a reboot may be in order.
Two minutes later: Yes, rebooting the host got openCL working again, systemwide and for darktable.
Something(s) new I learned while troubleshooting: LibreOffice uses openCL, too. And, separately, there is a clinfo package and tool that shows the overall status of openCL on the system. I also learned that my nVidia card has 1,920 Cuda cores for processing.