I wonder how much of it is self-suggestion, or maybe the fact that it maybe melted and became a different consistency/density and therefore the vaporization effectiveness changes (maybe some burnt and thats why it tasted bad)?   Or maybe some oxidized and this increases the bad taste (but n-oxide seems perfectly active)

In any case as said before better store in glass vial next time. But yeah you could try doing an A/B on it or just recrystalize, it should work fine. I suspect even if some oxidized, it will be a relatively small amount on the surface (because n-oxide is oil so if most of it had oxidized it would all be an oil).

And by the way, I know you've read this before and I've said this a million times, but there is a good reason so I'll say it again: Consider getting a vaporgenie and a torch lighter. Ideally the GVG but the other can work fine too. It will make the vapor sooo much smoother and effective, n-oxide or not