I have an article which is 52 pages long which I am trying to save in K2 but it won't let me do so. What I am getting is this error:

Error 325 (net::ERR_RESPONSE_HEADERS_TOO_BIG): Unknown error.

Though I still have this article in my back end and it shows all the content thereof in the editor window, but whenever I hit save or save&close it shows me the above error in Google Chrome and when I do try the same with Mozilla Firefox nothing happens.

Is there a way to create articles with unlimited number of pages?! It saves ok in com_content (though pagination doesn't work beyond page 10; links change from ?start=1,2, ... to ?page=2).

This will need testing.
Did you create one k2 item with pagebreaks that is 52 pages long?
Is there a readmore in the page?