As a general rule of thumb, I'd say anything which took place publicly and in the presence of a general audience can be considered common knowledge: for example, Jim's very public abduction, the expedition and homecoming etc. If you're not certain whether a particular event qualifies, it's best to check with one of the players involved.

With regard to Reynard's involvement specifically, that's definitely fair game: quite a few characters have been openly pinning the blame on him and it's safe to assume plenty of NPCs remember his previous attempt to trap the Nexus in permanent winter. Whether your characters choose to buy into that explanation/rumor, of course, is up to you. I hope that's helpful!