Can’t insert caption into image

I solved it, the problem was a function I used to use and forgot about!

 add_filter( 'post_thumbnail_html', 'remove_thumbnail_dimensions', 10 );
 add_filter( 'image_send_to_editor', 'remove_thumbnail_dimensions', 10 );
 function remove_thumbnail_dimensions( $html ) {
     $html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );
     return $html;
 }

I removed this function and everything is OK now.