I developed the addon on OSX. I got the same error from time to time but I read online that it's a problem with XBMC. A restart of XBMC should help.
A restart will allow the addon to start, but the only option presented is "Check your login credentials". I have verified that the credentials in /Users/user_1/Library/Application Support/XBMC/userdata/addon_data/plugin.video.nhl-gamecenter/settings.xml work on the actual NHL Gamecenter website.
Here's a log from the attempt: http://pastie.org/3169301
UPDATE: Same issue on my Linux HTPC. I can log into the website with the same credentials. What else could be causing this? Thanks!