Even though I have set-up a 4 hours sessiontimeout period in Moodle settings, my users were thrown out of their sessinos in 24 minutes.
I searched the moodle.org web-page and found a soulution on this page: Using Moodle: CRON job and sessions relation.
In short: the problem was that PHP setting (php.ini) session.gc_maxlifetime was set-up to 1440 seconds (24 minutes). I changed this to 3600 seconds (1 hour) and now it works.