Hi Ivica. Upgrading from moodle 1.9 to 2.5 is a HUGE step forward!
Did you follow the instructions to be found on the Questionnaire plugin page?
"NOTE: If you are upgrading to Moodle 2.3 or greater, make sure you have upgraded the questionnaire module to the latest version of Moodle 2.2, in your Moodle 2.2 codebase before upgrading to Moodle 2.3 or greater."
Joseph