I think I figured it out.
The underlying reason for the error seemed to be line 3390 of TweenMax.js:
_createSVG = function(type, container, attributes) { var element = _doc.createElementNS("http://www.w3.org/2000/svg",type),
This apparently was interpreted as cross-site scripting and therefore prohibited by the Flash HTMLLoader.
When changed to
var element = _doc.createElement(type),
the error disappeared, and everything worked fine - TweenMax.js loading, animations running...
...apparently without side-effects (did not explicitly try any SVG though).