I accidentally deleted the default modxcms.com add-on provider. I tried reinistating it with the url http://rest.modxcms.com/extras/, but now it complains with:
This Provider requires an API Key to connect. Please specify one.
I deleted and re-added the provider as you described, and I am having no problems connecting. It does not require an API key. I left that field blank when I recreated the provider.