The problem was one of the lines in css (probably ligatures or hyphens). When I deleted those lines (except font-family: serif

and recreated ePub and then converted to KePub (in Calibre), the problem was gone. I'll experiment more later in the evening
Font was not the problem, because it appeared with every font, when i change size above 20%, and on almost every page.
And davidfor was probably right that it appeared on one word sentence.
Thanks everyone for suggestions