Update Page function doesn't save and deletes page content

I’m working on a site for a customer that is currently on a temporary URL and has worked fine with no problems until now. I just upgraded to WP 3.5.1 (it was 3.5) and now I can’t update the pages – when I change some page content and click ‘update’ it deletes most of the page (except the first line or two)!! No errors, nothing – just deletes the content!

Have cleared cache, disabled plugins and also reverted back to default theme – problem still persists. So it’s obviously not plugins or theme!

So frustrating – any help, advice etc? I don’t want to give the URL as it’s a temp one. But surely that shouldn’t be causing it as it was fine before the update.

[No bumping. If it’s that urgent, consider hiring someone.]