>>>Use the developer tools in the browser to see what is actually getting sent.
>>>
>>It actually helped to figure out the problem eventually. The http call was wrong from that page
>
>Getting a 400 from the WebAPI controller was a good indication of that :-}
>
> - not sure how it worked before. I made a fix and it's now working.

I compared the APIController's code versions from TFS Source Control. I found that there was an extra route and method called GetLayouts which was commented out by my colleague and then removed. That's what broke it.