result: 16 means some data is missing:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/server.cpp#L93
I am not quite sure why you are getting this return value. You said your firmware is 2.1.8, but do you see a minor revision, like 2.1.8 (3) is minor revision 3? I could be that you are on an earlier minor revision of 2.1.8 that doesn’t support the en= parameter in /cp.