parse error after entering an API key

After activating the wordpress.com stats plugin and entering an API key, I get an error message:

Error from last API Key attempt:
parse error. not well formed

I know it’s not the issue with the API key, theme, or other plugins, because I have 6 sites that have the same theme and plugin set, and I only get this error for one of the site.

I saw two previous posts about this from years ago without any solution. Any help is appreciated!

http://wordpress.org/extend/plugins/stats/