Hi Richard - sounds like you might have a corrupted plugin preferences file. Please try these steps to see if it clears the error (don't worry, your devices, triggers, control pages and whatnot will not be affected.) Be sure to copy your API key before proceeding.
1. Ensure that the plugin is disabled.
2. Stop the Indigo Server.
3. Browse to this folder: /Perceptive Automation/Indigo 6/Preferences/Plugins/
4. Delete the file named: com.fogbert.indigoplugin.wunderground.indiPref
5. Restart the Indigo Server.
6. Re-enable the WUndergroung plugin.
7. Re-enter your API key and plugin preferences.
Hopefully, this will get you righted. You will have to re-enter your plugin preferences, including your API key, but it will retain all of your weather devices and the things that link to them (like triggers and actions.) If this doesn't solve your problem, we can delve more deeply.