13 means that xboxdash.xbe can't be found or isn't actually the ms dash xecutable
21 means that some of the files for your dash have been messed up
i would suggest that you just ftp into your xbox, delete everything off the c partition, thn transfer over a backup of the original c partition (if you don't have one find one) then ftp in your dashboard while you are there
what more than likely happened is that your evox dash's xecutable was named as xboxdash.xbe, you ran the update and it tried up update your evoxdash with the xbox live disk
when live first came out this was very common with people, but now since you DON'T need to have renamed any files it is less common
anyway do as stated above