This is a weird one, and though I solved it by trashing preferences, even THAT was a bit of an adventure.
I was happily working on a pretty large project, for which I use RX and Post Filter almost constantly as offline processes for notching out wandering ambient whines.
I call up Post Filter with the key command I've assigned... and it goes straight to processing the selection, without bringing up the plugin interface. Curious, I check some other plugins offline, and they all do the same thing. No interface; straight to processing. I restart. Doesn't fix it. I pull "defaults" from the prefs folder, and start Nuendo again, letting the defaults rebuild. Offline plugins then work as intended.
Ok, all my previous settings are trashed, but I've saved them as prefs-- but the meter colors don't come back with them, which sucks. So I think... go to backups from a month or two ago. Try that out. I do... and the problem returns, even though I have been working the same way for at least a year and the first time I've had this problem is... today.
Anyway, I ultimately re-rebuilt prefs and re-sort of-created my meters. I post here:
1) To register the issue
2) To the experience and resolution for others who may find themselves in a similar confounding situation.
3) To suggest that METER COLORS ought to be savable like the rest of personal prefs. Or... am I missing something?