You won't believe what the error was.
It was being caused by this line console.log("here"); in the following function:
Just remove it and you're good.
I think that has got to be the biggest face-palm moment of this year so far.
It's common knowledge that console.log is bad news for IE, but until I spotted this, I was testing on three different machines and had to result to setting up a VM, just to be able to reproduce the error.