Can you give is a reference for what you're referring to?

That I know of, no serious scholars have ever claimed to have "great" independent historical evidence of a miracle during Jesus's life. That said, there are a wide variety of attempts to lend at least basic credibility to miraculous occurrences.

For example, take 1 Corinthians 15:6, which states that after his death Jesus appeared to more than 500 followers at the same time.

In one of his "The Case for..." books (I forget which at the moment), apologist Lee Strobel claims that this story pretty much has to be true, because anyone living at the time could verify the story. I don't know any historian who would consider that defense as valid, but it's an example of "evidence" out there in the wild.

A better example, although one which I think still has problems, is presented by a gentleman named Keith Thompson here. He argues at length that Matthew may offer a second reference to this appearance. Multiple attestation is a valid historical argument, but while it is possible that Matthew references this story:

1. I think the evidence is poor at best, although not necessarily without merit
2. We have no way of knowing whether Matthew is an independent source for the story if it is in fact a reference, given that he almost certainly borrowed from earlier sources including Mark and Q, and possibly others. Mark and Q are later sources than Corinthians, and Matthew is likely later still. Even if Matthew is a second reference to this event, it's possible his source is Corinthians itself, Q, or some other source.

Anyhow, that's one example of historical evidence for a miraculous occurrence, albeit maybe not a great one.

I assume they refer to the biblical texts, which make these claims.

Ancient texts are full of miraculous occurrences. Historians used to assume ancient people were superstitious and credulous, and just wrote this stuff down because they heard it and believed it. Open an older book on, say, the Goths, and the author will completely ignore all the miracles and signs his sources describe to focus on the political events.

Now days, historians have realized this kind of nit-picking is methodologically problematic. We can't pick and choose the 'facts' that we like from an ancient text and throw out the ones we don't - that's not an objective way to learn about the past, it just reinforces our preconceptions. So a lot of recent scholarship, particularly in the field of Late Antiquity (early Christianity / late Roman empire / early middle ages), takes miracles seriously. But not necessarily as things that actually happened. Let me clarify with an older example. Ramsay Macmullen looked at early stories of conversion to Christianity, and found that conversion usually involved large groups of people who, together, saw a miracle, and signed up to the new faith together in response. He wrote that we have to take the miracle seriously - it's a key part of the story. But we don't have to accept that it was real. Rather, we can learn that miracles were important for early converts as a way of explaining their experience. 'They...took miracles quite for granted' (Macmullen 1984, p. 22). If we want to understand how early Christians understood their faith, we have to realize that, for them, miracles were a real and powerful part of the world.

Did these miracles actually happen? Depends on whether or not you believe the witnesses. Most modern historians assume that these miracles didn't actually happen, but that the belief in them was very real and important, and that belief could have real consequences for historical events. The evidence doesn't allow us to say more than this.

So when people say there's historical evidence proving Jesus performed miracles. The texts clearly describe these events, and they were clearly important to the early church. This is as good as the evidence we have for many other signs and wonders in the ancient world. Historians wouldn't consider this proof the events actually happened. But it does indicate that miracles were important, and that many people in the early church believed they were real. Whether or not you choose to accept their beliefs as more trustworthy than modern post-Enlightenment inspired skepticism is a personal decision.

(I will add, as an addendum, that my own research investigates some technological processes for iron working that look very unscientific and superstitious, but which actually were very logical and sensible within the practices of early medieval smiths. It's easy to write off ancient / early medieval beliefs as being wrong because they lacked science, but reality is often much more interesting and complicated. Ancient people understood the way the world worked differently than we do, but that understanding was almost always just as sophisticated and, in many ways, effective as our own; miracles are hard to believe in, but that's partly because our understanding of the world - our ontology - is so different from that of the Romans.)

This might also be a good question for /r/AcademicBiblical.

I don't know what you are talking about really, but miracle claims weren't rare in the Roman world. See Miracles in Greco-Roman Antiquity: A Sourcebook for the Study of New Testament Miracle Stories.

You should probably ask /r/AcademicBiblical.