Ninja Again I just checked the helper file and it clearly wrong.
Lines 324-328
$timezone = new DateTimeZone( JFactory::getConfig()->get('offset') );
$jnow = new JDate('now');
$jnow->setTimezone($timezone);
$jnow = $jnow->toSQL(true);
$dateok = $jnow;
So '$jnow' is now a string based on these lines and is use as such in the query lines 330-342. Then the next time '$jnow' is used is line 1064 and not reinitialize before this line.
$dateok = $jnow->toSql(true);
This code is trying to use '$jnow' as a joomla datetime object and it currently a string so this causes the 'Call to a member function toSql() on string' error. You can't make a object into a string then try to use it as a object later in the code.
Your testing version must have this error as or you have different code to the released version.
Also I see many people are having the same issue as per https://www.joomlart.com/forums/d/3491-adagency-6-0-19-upload-crashed-website/15