Under the laws of the Tzardom of Kislev, it is illegal for men to wield magic.
It didn't used to be italicized like that, because there was no need to. But about three hundred years ago this guy named Finubar began his tour around the Old World, and suddenly there was a lot of money hanging on the specific legal interpretation of whether the law referred to "males" or "male humans". The Tzar at the time, after careful consideration and debate (And Finubar dropping a shipload of Cathayan silks and Indese spices on his lap and saying there's more where that came from, y'know, for any port willing to do business) came down in favour of the latter interpretation.
As for why Kislev bans male spellcasters, Kislev has a long-standing cultural tradition of considering magic to be a feminine art. They believe that magic corrupts those who are ambitious, warlike, or greedy, and since those are all masculine traits, male spellcasters are dangerous. There is also an Ice Witch prophecy regarding a male Ice Witch tainting the flow of Ice Magic through Kislev, but I do want to note that this attitude and opinion is not limited to the Ice Witches. There are also no male Hag Witches, and they've held that tradition since before there have been Ice Witches, let alone any prophecies involving them.
They weren't super happy about male elves being allowed to cast magic, but trade with Ulthuan is incredibly lucrative, and it's hard to argue against it when the Phoenix King's ambassador is all too happy to supply estimations of just how many additional rotas Kislev has raised with that money whenever someone starts making noise about it. Over time, things have mostly settled down since no great catastrophe befell, and people have mostly figured that elves are different enough from men that they operate on their own rules, though it's still advised that male elves traveling in Kislev avoid openly casting spells if they don't need to.
And yes, it is illegal for men to wield any kind of magic, not just Ice Magic. If you can demonstrate you aren't actively wielding magic, just have the capacity to use it, you get Pacified rather than executed, but that's about it.