Hello Hax,

Thank you very much for your answer. I followed your advice and made some tests in order to determinate where the problem exactly is. It turned out there is confrontation between “Popular widget” and “WP Minify” plugin in the newest WordPress version – 3.4. The both plugins work normal together with some older WordPress version. I have already disabled WP Minify and Popular widget works fine.
Greetings
Serge