The API key is only needed to download data through their API. The upload protocol has not changed.
1, Are you sure you have the right station ID and password? It looks like an authorization error.
2. WU can be very unreliable. Are you getting the error consistently? Or, just every once in a while?
3. You can try to post using their sample URL. This will post a value for 1-Jan-2000, so it is unlikely to corrupt up your WU station data. Take the following and paste it into your browser. Make sure it is all on one big line.
Then replace the station id KCASANFR5 with your station ID, and the XXXXXX with your password. Then hit Enter on your browser. It should say "success". If it says "unauthorized" you have an authentication problem.
-tk