Recently I happened to use SPCalendarView controls in SharePoint 2013.
We do not have a root site collection created but have a sub site collection.
We have www.xxx.com/sites/site1 but we do not have www.xxx.com root site collection.
When SPCalendarView control is used, it refers to images pointed to the root site collection. This error is also caused by GetThemedImageURL() method. Here is the error I got first time.
As nothing stops us from creating a root site collection, I just went ahead and created this site collection and voila, Calendar popsup.
If you goto, http://yourwebsite/_layouts/15/iframe.aspx?&cal=1&lcid=2057&langid=1033&ww=0111110&fdow=1&fwoy=0&hj=0&swn=False&minjday=109207&maxjday=2666269&date=01%2F01%2F1980, this should render calendar successuflly.