Other users' Public folders - sometimes can upload, sometimes can't!

[Let's Sekuhara!]

Recently I uploaded a bunch of photos to a friend's iDisk Public folder.
A few days ago when I tried to upload to a different friend's Public folder it failed. It failed both when attempted through Finder and through the web interface. When I try to drag to the file in finder I get a cursor icon of a circle with a strike-out through it. Has anybody else experienced this?

Thanks.