Hi, there isn't a way to know where this error 21 come from. We realised that this error came often when a client just buy a MPPT to us, not a full solar kit. And this happended when there is many solar panels or batteries.
The last case, the client has 6 solar pannels 160W 2s 3p, 960W and 5 batteries in 12V for a MPPT 100/50. (Already told this client to improve his install before change the MPPT)