Filter boxes suddenly stopped displaying in IE11 (works fine still in Edge and Chrome)

Jul 21 at 11:52 AM
Title says it all really. No code changes that i'm aware of in the site (WSS 3.0).
We use this list filter in a number of lists across multiple sites and all stopped working at the same time.

Is there an update/patch to IE or Winodws that has broken this customisation? I am using the exact code as per the download link here:
https://instantlistfilter.codeplex.com/downloads/get/63332

Pasting the code into a CEWP.

Most of our users are on Windows 10 if this makes a difference.

Anybody else experienced this issue?
Anybody have any pointers on where to try debug this issue?

Have tried an IE reset and clearing cache but to no avail.

Any help gratefully received!!

Thanks,

Alex
Jul 31 at 11:39 AM
Further info: Interestingly, this customisation works ok in Edge and Chrome but not IE.
Annoyingly we need to use IE for some of the other SharePoint related stuff so we're a bit stuck now!