Server error at large posts

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http://serverfault.com – On a large drupal site, the database server is on a separate server connected directly to the web server. The web server uses apache and memcached. The problem is that whenever the post is a large, say larger than 10KB, the server does not return correctly. I checked both apache and mysql logs but could not find any trace of errors being logged. The error happens also when I use nginx/php5-fpm instead of apache. Despite this, the large posts are registered, however incorrectly so that they show up for admin when I open a new page on the site. I'm really confused and appreciate your hints to (HowTos)