well, as i'd the same problem a couple days ago, i can recommend you to delete this 0kb nand.bin file, backup the sd (if it had another content), and reformat (i recommend you this because when this error happened to me it, even after deleting nand file, the free-space of my sd were >500mb lower - if you are using a normal sd (capacity <= 2gb, no sdhc logo), i suggest you to format as fat16)
then try to backup again, but now you better check 2 things:
1. after backup try to use the restore app, it will run in simulation mode first, if this mode recognize your nand.bin file, your backup was correctly made (you can cancel restoring after checking this)...
2. also, your computer should recognize this file with a >500mb size...
if after reformat your sd, it still giving you this problem, you better wait to a newer release of bootmii as they are now working to correct this issues.. or if you can't wait any longer to backup, try with a diferent sd.. wish you luck using this..