Bug with SVG A elements
I had to track down a bug when trying to install AddThis widgets on a page using SVG.
The function getcbn in core004.js is looking for <A> tags with a class containing the addthis_button string. However, the field className does not return the "class" attribute of the element in SVG but something else. Therefore, the indexOf(k) raises an exception.
You might want to check that the returned "A" elements are HTML and not SVG to avoid this bug.
It was preventing AddThis to work actually.
Can you link to an example page or post some code? That would make it a lot easier for our developers to fix.