I keep trying to upload images to Flickr from iPhoto via FlickrExport but I get "Error 2 in photos.upload.checkTickets."

Seems like is it on Flickr's side. A Flickr help post has a message from staff on this issue:

Hi folks -

This morning we lost a part of Farm1, and as a side-effect we've stalled on the uploads. We're working on the problem right now, and we're hopefully going to get it sorted out ASAP.

Sorry about this!

Hope it is fixed soon, got to show you guys something. :)