According to this thread it seems to me that the majority of the answers say that に間違いない is a stronger version of に違いない, while に違いない is closer to a personal "sureness". I'll paraphrase some of the answers for convenience.
According to the top answer, に違いない means that the speaker came to a conclusion and is very sure that it is correct, but it is ultimately their own opinion. There is a little uncertainty, but the speaker is very confident in what they are saying.
に間違いない is described as a 100% certainty that there is nothing wrong with the statement. It seems to describe a personal conclusion as well, but is a bit more objective.
One of the other answers explains it like this:
So, に違いない ＝ にほぼ間違いない.
I don't know if there is a maybe more nuanced difference between the two, but this seems to be the general agreement on the internet. Some other sites like this HiNative page say similar things.