So going back to the OP, I'm thinking another potential lesson from this experience (other than fixing a flawed beer) would be learning what went wrong in the first place, and how to avoid it in the future. Did you come away with any lessons for future brews?
This beer was brewed exactly the same way that all of the past beers have been brewed. My procedure is very standard, with little variation from beer to beer.
At first, I thought it might be a yeast issue, having harvested this Diamond Lager so many times that we have lost count.
Also, my recipes are very standard. The one thing that was done with the 20 gallons of 5D (German Pils) that I have not done previously is a Diacetyl rest.
Not a hint of any off flavor at all. No Diacetyl. None.
So I might incorporate a D-Rest with all of my beers going forward.
But here is the question...exactly when do you do this? At what temperature? And, for how long? Do you take gravity readings at this time?
Like they say, an ounce of prevention is worth a pound of cure...or yeast, in this example.