There is a problem with Drupal 6 site. It seems seesion cookie is preventing some pages to load.
Site has approximately 5 to 10 pages (out of 30 000) of difference content types which can't be loaded. No errors returned — browser's wheel turning endlessly. After disabling cookies pages load just normally. After some testing I can say, that the problem in session cookie.
The problem doesn't exist in the development environment. I have made some testing on production (stopped Nginx and restarted Apache as single web-server) to make it similar to to development environment, but nothing changed. Now I have no idea how to debug the problem.
Thanks for any pointers.
The problem was in big sessions table (2GB). After truncating the table error has gone.