I don't think it'll help, but just for the sake of completeness: you could update to version 4.0.1 and check if the error is still there.
But I think that sounds like the beginning of a long and difficult journey. So be it!
https://mozilla.github.io/pdf.js/web/viewer.htm is the ES2015 version. You're right, I have to update my bug template. The version for IE and Edge is https://mozilla.github.io/pdf.js/es5/web/viewer.html.
I suspect the problem is caused by a font embedded in your PDF file. I'm not an expert with respect to the details of the PDF file format, so it's just a far shot. Let's try to create a reproducer first.