Fatal error: wp-db.php on line 2344

Hello,

I can’t display pages list on a WP install :
Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 71 bytes) in …../wp-includes/wp-db.php on line 2344

Any idea?
I allready :

• desactivate and remove all plugin.
• have other websites run with the same server settings, nothing specific on this account
• use a default theme (i get error on line 2350 in this case )

Maybe I have to clean the pages from the databases ?

thanks for help