Could be the ejector...
When you mentioned the 'pin' I had to think about it for a second or two... There is the round ejector 'button' or 'pin' which stands proud of the bolt face - but off center.
I know there's a wicked spring that holds that sucker in place - I suppose that it could be frozen, but perhaps it's just stiff?
I would take some brake parts cleaner, or some carb cleaner, and spray the bolt liberally with this, allow it to drain off and dry, and then try to load and lock the weapon. If this works, you're good to go. But if it doesn't, it's time to disassemble the bolt and troubleshoot. And no, I don't believe I've seen or heard of this problem before either.
I would imagine that the stuck ejector is probably your problem, as if the firing pin were frozen out, you should still be able to drop the bolt home, there's just a chance that the rifle would fire as the bolt went into battery. A slight chance I would think, as it usually takes quite a smack to detonate a primer.
Let us know how you fare.