Weather Underground discontinued all of their free api keys as of Dec 31st 2018. People who were supplying data to WU from a personal weather station and using a free api key were supposed to be exempt, but it seems like they’re being offered access to a different api instead. SmartThings is in the process of moving all of their weather to an api from The Weather Channel. Here’s one of the current discussions about this:
https://community.smartthings.com/t/weather-api-changes/147511