Unable to add any files to download. (BB-3122)

I tried several times adding files to the download section. I'm just unable to.

Once I select the file and click the button, the request gets transferred to amazonaws. But it returns an error.

The URL I land at after the error is:

http://hg.sterex.in/base/downloads/add?bucket=bbuseruploads&key=sterexin%2Fbase%2Fdownloads%2Fmodel.pdf&etag=%22a3280caca9c1dbb1732a0ee7ac03fb16%22

To note:
I use a custom domain. Should that matter?
I have attached a screenshot (of the kicked-over bucket!)