What have you done immediately before the box hung for the first time ?
How much images do you have in multiboot and which images ?
When the box hangs, is it reachable by SSH and OpenWebIf ?
I didn't do anythig before the box hung for the first time.
I never used multiboot.
The box is reachable by SSH, not By OpenWebif; after opkg update command, SSH says "Could not lock /run/opkg.lock: Resource temporarily unavailable".