An easy way to translate these correctly is "it's a matter of" and "it's not a matter of."
Er is spraken van een vergissing = "One could call it a mistake." In other words, somebody blew it but I want to use passive language that avoids saying exactly who.
Er is geen spraken van een vergissing = "It was definitely not a mistake." In other words, it looks like a mistake to many observers, but the speaker wants to deny it.
It's a lot like the French "question," which can also be found in English, and is used in the sense of 'issue.' As in: "it's a question of right and wrong." and "there can be no question of him being temporarily insane."
As an editorial remark, I find the idiom strangely annoying in Dutch. Its use is confined mostly to politicians.