Attaching an attachment to an issue fails

I'm trying to attach a document to an issue at https://bitbucket.org/poupas/python-inotify/issue-attachment/1/, but it fails. After having selected the document and clicking 'attach' the page turns blank. Reloading the page shows that the attachment was not uploaded.
I'm using firefox 24.0 on ubuntu linux 13.04. Using Opera 12.16 also fails, it tells me "The remote server refuses to perform the request. This address is not available." The adress in the url bar does not change.