Cannot authorize at VKontakte

What is the URL of the page that gives you error 400 or error 404?

It was something like
https://oauth.vk.com/err400

But today it is different. I get the page
https://oauth.vk.com/error?err=2
and it says:
{"error":"invalid_request","error_description":"Security Error"}

Updated to 3.1.0, still the same…

After searching for a solution and many fruitless attempts to fix the problem I stumbled upon an advice: “try using another browser”. Whaddayaknow? I tried the same link calling vkontakte API with exactly the same parameters and all – and it worked. Weird.