Memory size exhausted after ~15k pages generated

Hi,

I’ve generated around 15k pages using the pro version and now I am unable to edit a page or edit the theme customizer because of a Fatal error: Allowed memory size of 268400456 bytes exhausted (tried to allocate 38122 bytes) in …/wp-includes/wp-db.php on line 1924.

I’m running a VPS with plenty of memory available.
I’ve changed php.ini and wp-config to 256M, 512M and even 1024M, but the error persists. I’ve also disabled all plugins, tried a standard theme etc.

Could you please give me more info on how the pluging works and if possible a suggestion to fix the problem? Thanks