Invalid JSON response when adding some utf-8 characters

Hi, I have some problem with DataTables 1.10 and server-side script. Everything works well when I import to MySQL table CSV data without German and Polish characters, but when I add some of this specific characters, I instantly receive an Invalid JSON response error. All of settings in phpmyadmin database is set to utf-8 as well as main php file. I would be gratefull for any help.