Just thought I'd mention that I was able to add new network connections after reflashing the device. I still don't know what might have cause the problem in the first place. It sort of feels like an out-of-space error, but I had 8Mb free, so that shouldn't be it — unless the number reported by df is inaccurate, or the filesystem the N770 uses requires a lot of space free in order to update a small file. So I'm still puzzled how to avoid this problem in the future.