Can't add new posts: memory error

I am currently unable to add new posts. Clicking Post > Add New shows the following error:

Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 491520 bytes) in /var/www/www.mysite.com/wp-admin/edit-form-advanced.php on line 306

For testing purposes, I have the default template (twenty-eleven) running with all plugins disabled and the error still shows.

I have increased the memory to 512M in php.ini, wp-config.php, and in default-constants.php, but I still can’t seem to get past this error (which seems to show an allowance of only 256M).

Just FYI….the database is not new – it has over a thousand posts in it and we recently upgraded to 3.5.2. This issue only occurs for certain admin users, others can see the Post > Add New screen.

Any help would be greatly appreciated. Thanks!