WSOD wp-admin/edit.php after 1500 pages

on three different websites I experienced the same problem:

passing 1500 published pages I get white screen of death towards 2K pages when loading the ‘All Pages’ list. It seems not to be a question IF it happens but WHEN. And definitely before 2K pages.

The websites are hosted on a dedicated server. I have set all php limits already to unlimited in wp-config.php and in two php.ini files (via Plesk Panel and SSH root access).

The sites are running through Cloudflare with nginx and varnish cache. I tried with and without those three services, reloaded almost any webserver configuration. But white screen remains.

Until white screen happens there is no slow down or any other problem of editing pages. Revisioning is disabled completeley since publishing the first page. Editing single pages and updating still works normal even after white screen appears to load the whole page list.

Deactivating plugins should be a later option only if necessary. But I don’t think so. I have activated 75 plugins I do need almost every single one and they are tested for quite a long time in many configuration steps.

Published pages have a regular parent/child hierarchy max three levels down. A tree view mode of all pages works properly.

It might be a theme related problem. I am using newest iThemes builder custom themes but never experienced any trouble. Before I contact their support I wanted to be sure there is no WP related issue that might be the reason for the described problem.

Any thoughts where to start troubleshooting. Disable further functions?

Thanks

Ruediger