Iv ran into this problem as well, Im running fog 0.32 on ubuntu 11.10 32-bit. I had this issue fixed at one point and did that by doing a complete reinstall of the server but i would rather not have to do that again. For me i can multicast with no problems and upload without issues, this only happens when i attempt to push an image via unicast. Also this is not isolated to one of my images, it happens to every image i have saved on the server.
One thing i noticed is my ubuntu desktop gui froze last night but the fog server was still up but running extreamly slow, but both unicast and multicast worked. I did a Hard shut down and restart and now i get this same error on every image.
Not sure if this helps anyone figure out what is causing this problem but ill post again if i find any solution besides a server reinstall.