SOLVED…for the benefit of anyone else with a similar issue…
The problem was cause by using the “specific folder” options for the global preference “setting location”. This allows us to synchronize preference settings across multiple computers on a network. My guess is one computer encountered a problem opening the “Wavelab Plug-in Set.vst3” VST plug-in and somehow it was automatically added to the “do not load the following plug-ins” list, a setting which is shared across all installs on the network. This appears to be not overridden by the “ignored plug-ins” selector box in the plug-ins organize tab, which allows you to re-evaluate ignored plug-ins. Once I removed it from the “do not load the following plug-ins” list all was good.
Sharing the settings across a network is a great feature, as long as you’re aware that some settings might change without you realizing it.