Hi, i guess you have to change some code in the *.js file of mmm-openmapweather.
there is a row with //,“50n”: “wi-night-alt-cloudy-windy”

due to the point that there is no image for the “fog at night” i used the regular fog image.
now it works for me , yes it is not 100% correct, but better than nothing.

iconImageTable: { "01d": "sunny.png", "02d": "partlysunny.png", "03d": "partlycloudy.png", "04d": "cloudy.png", "09d": "sleet.png", "10d": "rain.png", "11d": "tstorms.png", "13d": "snow.png", "50d": "fog.png", "01n": "nt_clear.png", "02n": "nt_cloudy.png", "03n": "nt_mostlycloudy.png", "04n": "cloudy.png", "09n": "nt_sleet.png", "10n": "nt_rain.png", "11n": "nt_tstorms.png", "13n": "nt_snow.png" ,"50n": "fog.png"

@varad-wattamwar i posted in your github issue, you determined it was a dialect problem and I gave you options for trying to resolve it

just as Mykle mentioned, posting in multiple places does not make an answer appear faster