I've noticed a strange bug on my site, which also happens here on XF (using IE11).
I can paste text in to the editor fine in most conditions, but if I insert quote code and try to paste between tags, nothing is pasted. Something happens, as the editor flashes for a second, but nothing is inserted.
I believe this should be fixed now. I'm not 100% clear on what actually causes it (well, why it's happening). IE is refusing to use the range that we pass it, so it loses the selection. It then proceeds to error because we try to get an invalid range... except it doesn't actually print the error to the console. It just stops. Oh you, IE...
I've worked around this case as it involved a collapsed selection, so a lot of the modification work isn't necessary, so we can just skip the whole bookmarking/restoring of the selection that triggers this issue.
This fix should be rolled out now, though it might need a hard refresh.