Unspecified Error in Office Addin Dialog Window

I'm developing a Word Addin for Word 2016 and getting a rather unhelpful error in the Dialog window; Unspecified error. It appears to be occurring at a history.replaceState call but I'm having trouble figuring out why.

The Dialog window contains a web app in an iframe and seems to break when it attempts to load the web app. I have a stack trace:

Error: Unspecified error.
  at replaceState (https://localhost:4200/assets/vendor-63142ea35c982dab5ee087d2647e494f.js:43050:7)
  at initState (https://localhost:4200/assets/vendor-63142ea35c982dab5ee087d2647e494f.js:42930:7)
  at applyStr (https://localhost:4200/assets/vendor-63142ea35c982dab5ee087d2647e494f.js:58464:7)
  at tryInvoke (https://localhost:4200/assets/vendor-63142ea35c982dab5ee087d2647e494f.js:58954:7)
  at Anonymous function (https://localhost:4200/assets/vendor-63142ea35c982dab5ee087d2647e494f.js:42582:7)
  at _setupLocation (https://localhost:4200/assets/vendor-63142ea35c982dab5ee087d2647e494f.js:46600:11)
  at setupRouter (https://localhost:4200/assets/vendor-63142ea35c982dab5ee087d2647e494f.js:46242:7)
  at startRouting (https://localhost:4200/assets/vendor-63142ea35c982dab5ee087d2647e494f.js:46227:7)

The web app works in IE11 and Edge - just not in the Dialog window of the Word 2016. I should note the Word Addin works in Office Online and Word for Mac (v15.30).

My question is: does anyone have any intuition or insights as to why/where this might be happening? I'm completely stumped. Anything to point me in the right direction?

Sorry it's all a bit vague but that's all I've got to go on - hence, why I'm having trouble. I'm using Windows 10 and the web app is Ember 2.13.3.