I've experienced upload failures on Wikimedia Commons both today and yesterday (on using UploadWizard) and we are getting reports of similar issues from one of our (Wikimedia Sverige) workshops today.
Note that it doesn't seem to stop all uploads (I succeeded on one later in the evening yesterday), however I also got the same error (with the same file) on http://commons.wikimedia.beta.wmflabs.org yesterday.
The pattern is that you go through all of the UW steps and on submitting it it fails to load the use screen or any progress bar (see screenshot).
Inspecting the console only gives me the below (unsure if it is of use):
TypeError: this.copyMetadataWidget is undefined uw.controller.Details.prototype.submit() load.php:866 uw.controller.Details.prototype.startDetails/<() load.php:864 jQuery.Callbacks/fire() load.php:45 jQuery.Callbacks/self.add() load.php:45 uw.controller.Details.prototype.startDetails() load.php:864 oo.EventEmitter.prototype.emit() load.php:578 startDetails() load.php:875 oo.EventEmitter.prototype.emit() load.php:578 OO.ui.mixin.ButtonElement.prototype.onClick() load.php:607 <anonymous> self-hosted:751 jQuery.event.dispatch() load.php:65 jQuery.event.add/elemData.handle() load.php:60 load.php:866:384