When my user tries to edit an article in frontend, he gets "403 - you don't have permission to access(#some id of article)" and when he goes back in browser and tries again usually he can get access - sometimes he need to go back and try again few times. Now this is not happening all the time. I can even recreate this by adding user to different access group and after this trying to access some articles. Then I get 403 blabla, but after few tries I can access the articles editor.
What causes this? Is this because of some cache? Joomla cache is turned off, Apache cache is on. Maybe this is connected with my other stack question: https://stackoverflow.com/questions/35009053/random-redirect-301-only-in-firefox or could this be caused by rare packet drops?
Also I wanted to post this on official Joomla forum but my ip at work is banned on that forum. I really appreciate any help. I also posted this on stackoverflow: https://stackoverflow.com/questions/35031929/joomla-random-lack-of-permission-to-access