I have added the missing space between the header [month year] and the [nextbutton].
Change this string in [cal-functions.php]:
jQuery("#ics-calendar-header-text").html("' . $today['month'] . ' ' . $today['year'] .'");
to:
jQuery("#ics-calendar-header-text").html("' . $today['month'] . ' ' . $today['year'] .' '.'");