At first glance seems like a promise rejection (we should have better logging of those). I tried to check with the code below whether any provider rejects, and didn't find anything, but maybe I messed up the code:

$.each( mw.mmv.provider, function ( providerName, provider ) {

var oldGet = provider.get;
provider.get = function () {
  oldGet.apply( provider, arguments ).then( null, function () {
    var d = $.Deferred();
    console.log( providerName + ' rejected!', arguments );
    return d.reject.apply( d, arguments );
  } );
};

} );

Apparently I did mess up; the error is caused by the user provider rejecting when the user is missing. This does not happen for local and beta sites (local users always exist, and API repos are queried via JSONP so again the user always exist), only for ForeignDBRepo images (although I imagine it could happen when a user gets deleted/renamed with just the right timing).

Change 116044 had a related patch set uploaded by Gergő Tisza:
Create fake user when userinfo request fails

https://gerrit.wikimedia.org/r/116044

Change 116044 merged by jenkins-bot:
Create fake user when userinfo request fails

https://gerrit.wikimedia.org/r/116044

Change 116506 had a related patch set uploaded by MarkTraceur:
Create fake user when userinfo request fails

https://gerrit.wikimedia.org/r/116506

Change 116506 merged by jenkins-bot:
Create fake user when userinfo request fails

https://gerrit.wikimedia.org/r/116506