An original copy of the MS dash will only run from the root of the C partition, and then only via "xboxdash.xbe". It is expected that you would get an error 21 when running it from your backup folder.
"msdash.xbe" is a hacked version of the original "xboxdash.xbe". It's rigged so as not to launch your softmod when you run it, and it also has the above-mentioned limitations removed. However, if you were to rename that file to "xboxdash.xbe" and stick it on the root of your C partition, I'd expect the machine to boot to an error 13/14, as it's not the original.