have you tried adding "rootfstype=ubifs" right before "root=ubi0:rootfs" ? I've seen you tried it yesterday, but what happens if you do same steps as today and add this one in front of "root=ubi0:rootfs"? or try to change "ubi.mtd=rootfs" to e.g. "ubi.mtd=0" (actually the last one may be stupid, I have no experience in uboot on n900, but i remember setting ubi.mtd as a number on beagleboard or some other development board, not as string... it's just a suggestion which you may ignore )