It happened with the update (upgrade) version 2.9.1 and now with 2.9.2. After the upgrade there was found with the site is not fully functional, even the administrative part.
The problem then is it due?
First lazy not to make the aggiornamneto according to the directions given by WordPress.org inviting you to turn off all themes and plugins configured on your site.
Specifically, however, this problem is given by the activation of the plugin that MyGallery not find two files generates this dreadful problem. The files indicted are:
In fact, reading the code of the plugin can be observed:
So you do not need is included in the folder wp-includes the site in question and the two files streams.php gettext.php and everything will magically work!
Greetings to all!