"Add Media" button stopped working- JS errors

Upgraded from 3.4.1 to 3.5.1 and everything seemed to work at first, including the “Add Media” button- now, it seems to have quit functioning.

The behavior of the button is that clicking it does nothing at all. Opening the inspector, and I get this error on click:

Uncaught TypeError: Object function (a){return new m(a)} has no method 'omit' media-views.min.js:1
f.controller.Library.f.controller.State.extend.initialize media-views.min.js:1
g.Model backbone.min.js:10
f.controller.State.Backbone.Model.extend.constructor media-views.min.js:1
d backbone.min.js:38
f.view.MediaFrame.Post.f.view.MediaFrame.Select.extend.createStates media-views.min.js:1
f.view.MediaFrame.Select.f.view.MediaFrame.extend.initialize media-views.min.js:1
f.view.MediaFrame.Post.f.view.MediaFrame.Select.extend.initialize media-views.min.js:1
g.View backbone.min.js:33
f.View.Backbone.View.extend.constructor media-views.min.js:1
d backbone.min.js:38
d backbone.min.js:38
d backbone.min.js:38
d backbone.min.js:38
wp.media media-models.min.js:1
wp.media.editor.add media-editor.min.js:1
wp.media.editor.open media-editor.min.js:1
(anonymous function) media-editor.min.js:1
v.event.dispatch jquery.js:2
o.handle.u jquery.js:2

On the page load, however, there is another error that shows before the one above:
GET https://site.com/wp-content/plugins/tabbed/js/ajax.js?ver=3.5.1 404 (Not Found) post-new.php:38

NOTE: The actual URL of the site in question has been obfuscated for confidentiality- there isn’t actually a GET request going to site.com ??

Thanks in advance for any help.