Just checked. Upload.wikimedi.org is asking for username and password, using basic HTTP authorization on old images (for example: http://upload.wikimedia.org/wikipedia/en/archive/5/54/20071203110014!Territories_of_Dynasties_in_China.gif )

Changing component, as this is a wikimedia issue, not a mwdumper issue.

maybe it only happens for enwiki?

http://upload.wikimedia.org/wikipedia/commons/archive/5/5f/20071231224517!Fiction.gif works

Hmm, doesn't seem to be all wikipedia images either. http://upload.wikimedia.org/wikipedia/en/thumb/archive/7/70/20070807064948!Example.png/111px-Example.png is fine, so is http://upload.wikimedia.org/wikipedia/en/archive/8/8f/20070224001032!Example.gif Seems to be specific to [[file:Territories_of_Dynasties_in_China.gif]]

Then perhaps it's one of old images deleted from a bug and recreated as folder.

In that case, the server should give either a 404 or 403. People browsing to http://en.wikipedia.org/wiki/File:Territories_of_Dynasties_in_China.gif get a lot of popups asking for there username and pass, which is rather confusing.

I'm not saying that a 403 there is right. Just trying to guess the reason. I think folders used to be listable but now they aren't. Thus, if it is a "folder image", and now (ie. server move) the server was configured to give a 403 when requesting folders, that would be a reasonable explanation for the new error.

PS: Obviously the solution is to delete it and send people to http://commons.wikimedia.org/wiki/File:Territories_of_Dynasties_in_China.gif :)

Yes, this is a directory with nothing in it, dated Sept 5 2008, so it's a remnant of the thumb generation bug we had for a while on that day. I should clear those out.

In the meantime the configuration issue has been fixed (wrong keyword in an acl).