What does error code 21 mean?

Answer 1

if this is for iTunes then here is the answer

this happenned to me when i attempted to jailbreak my 4g iPod with limera1n so this is what i did

when it showed to connect your ipod to itunes on your ipod screen i held down the lock and home button until the screen flahsed black, then i let it go. after this i had to wait a few minutes before it went back to normal.

its as simple as that