The fix seems to work on IE11, but as it turns out, our client needs both IE9 and IE11 to work simultaneously for some time. And we have this problem now: With fix it works in IE11, but not in IE9 and without it it’s reversed.
When you pointed out that drag and drop works only with images – was the non-working course running in IE9 or IE10/11?
And did you convert the objects to images by checking the option Convert to image or did you have to use images from the start?
These issues usually come back to the Compatibility Mode. Turning on Compatibility Mode can drop the browser back to IE7 and Quirks which just won’t run the functions correctly.
But it looks like you’ve found a workaround with the custom script and some browser checking. Hopefully most of this will start to disappear next year when Microsoft drops support for IE8 & IE9 and companies are forced to move up to a modern browser.