[Mediawiki-l] Navigating to a page and then pressing back during an edit loses edit

[someone somebody]

MediaWiki version: 1.15.1
PHP version: 5.3.0
MySQL version: 5.1.37
URL: N/A (localhost)

I'm using XAMPP 1.7.2 When I'm editing an article and then navigate to an
new page and afterwards press back the content that I edited reverts to the
version before the edit. Also when pressing back after pressing Show preview
I get a session timeout error.

This doesn't happen on the official English Wikipedia.

Can you help me fix this?