Issue occurred after migrating website from Joomla 1.5 to 2.5 (using SP Upgrade). When admin user was logged-on in the front end, on some pages following error was displayed:
Problem was caused by checked-out articles. As users were not migrated using SP Upgrade (website had only one user) checked out articles were marked as being checked out by super administrator from old 1.5 website (UserID 62), which didn't exist on the new Joomla installation.
Solution was to unlock all locked (checked out) articles. This can be done either manually in the back end, or if you have lots of articles, running following SQL statement against Joomla database (using phpMyAdmin or similar):
uytox_ Joomla DB table prefix.
62 - old Joomla 1.5 website user ID