This is definitely not a known issue for firmware 2.1.9(9). I am not sure whether it has to do with your Wemos. Probably the easiest way to set the parameter is to go to:
http://raysfiles.com/os/TestOSAPI.html
type in your OpenSprinkler’s IP, port number, password, choose the ‘Change Option (co)’ API, then
Option name: wto
Option value: {}
this will set an empty string for wto. Hopefully it works.