Ok, I got it sorted out, in the file fsCalendar.php, in the part:
static $valid_fields = array('eventid',
'subject',
'tsfrom',
'tsto',
'allday',
'description',
'location',
'categories',
'author',
'createdate',
'publishauthor',
'publishdate',
'state'
);replace 'tsfrom' and 'tsto' with 'from' and 'to'
