Sesskeys are a security feature that stops links like [siteurl]/user/delete.php?id=4&confirm=1 from working if they are posted, for example, in a forum (the above link is made up, but hopefully you get the idea). If the link wasn't generated by the current user's session, then the 'sesskey' won't match and it will be assumed to be a malicious link.
Which brings us to your problem. Usually, 'missing sesskey' errors are only seen if, a) you try to construct a malicious link to do something bad or b) a programmer has made a mistake.
As I don't think either or likely in this case, it is probably special case c) - there are so many form items on the page that your server has removed some from the list (this is a security feature to stop people overloading a page with form values); the removed values, in this case, include the 'sesskey'.
To fix - edit your server settings in php.ini and change the value 'max_input_vars' ( http://www.php.net/manual/en/info.configuration.php#ini.max-input-vars ) to something higher (there may be other security settings on your server that have a similar effect, but this is the usual culprit).