I was playing a game with my brother when the following situation came up:

I controlled a Crypsis (with 1 virus counter) and a Sacrificial Construct. I also had three tags, so I feared that my brother may try to trash my Sacrificial Construct. As a result, I made a run on one of his servers (HQ, if I recall correctly), and used Crypsis to break the subroutine on the ICE protecting the server. From there, Crypsis forces me to remove 1 virus counter or trash it. I chose the latter, sacrificing my Sacrificial Construct to keep Crypsis in play.

My brother claims that this is an illegal play, and that because Crypsis had a virus counter, the virus counter must be removed. I may trash it only if Crypsis had no virus counters on it to begin with.

I say that I still get the choice and can give up my Sacrificial Construct to keep Crypsis and its virus counter.

In the end, it didn't matter, because the card I accessed ended up being an agenda which gave me enough agenda points to win the game regardless, but for the sake of future reference, who's right?

PS If this situation sounds familiar to you, it is because my brother already posted this on reddit. I figured that I'd go for something more official, just to be safe.

The card doesn't say anywhere on it that you MUST use the Virus counter first. It says do one thing or the other, so you can choose.

It's like Infiltration: Expose one card or gain 2cr. You choose; you don't HAVE to expose just because there's a valid target.

It would be different if, for example, Cypsis said:

When an encounter with a piece of ice in which you used Crypsis to break a subroutine ends, remove 1 hosted virus counter. If there are no Virus counters that can be removed, trash Crypsis instead.

To touch on another topic from your post - this forum is no more "official" than Reddit, BGG, etc. The only official thing is the FAQ. Yes, that document doesn't cover everything, but it's the only "one true source" as it were.

That said google around - there's also Lukas' (designer) twitter feed, which often includes rulings which, while also not "official" tend to carry a lot of weight in arguments with fellow players.

And THAT said, 99% of the time, it seems the general consensus of the BGG/Reddit/this forum communities TEND to align fairly well. But, it never hurts to check 'em all.

I can see where the must use virus interpretation is coming from, but as others have said the option forced by Crypsis is that you must spend a virus token or trash it.

When using a Sacrificial Construct as far as Crypsis is concerned you didn't remove the counter and a trash effect takes place. That it will not be trashed because of the SC is immaterial to the options granted by Crypsis.

Take care to note that SC specifies "prevent". Prevent and Avoid are special keywords in A:NR that allow you to sort of 'break' the usual timing of things. Normally actions and effects resolve completely, these two keywords are the ones that allow an interruption. Actions and events which cannot be prevented - will actually say "cannot be prevented" (see http://netrunnerdb.com/find/?q=flare for an example).

Take care also - if a COST of a paid ability specifies something be trashed, you CANNOT prevent it if you still want the benefits of the paid ability. The example for this (in the FAQ) discusses Cortez Chip, which has a special effect when you trash it. If you trash the chip but then prevent that trash by using SC, you haven't satisfied the cost and therefor the special effect doesn't happen.