It might actually be error 21.
On my DP245 3.0 I was initially confused, too. It read
E-Code1: 21
E-Code2: 0
E-Code3: 0
and so on.
This doesn't mean "Error 1 happened 21 times" etc., the actual meaning is "first recorded error: Error 21, second recorded error: None", etc.
For DP245 display, error 21 is the speed sensor, you might check your display's manual, but I assume it's the same.
I don't think you need to worry if it was only one time.
