How to sync with EndNote Web

I am having the same trouble. I enter my account information under “Enable Sync”, click “OK” and then the window keeps popping back up (yes, I am sure my account information is correct; I tested it by signing into EndNote Web several times). If I click the red dot to close the window my account information appears in the Sync Preferences pane, but when I click the sync button I get the following error message:

Sync Details
Sync was last run: Never
Sync Status: There are changes that need to be sent.
Sync Errors: Server.AuthenticationToken.invalidClientCredentials

Local Sync Library
Library Name: My Library.enl
Records: 0
Groups: 0
Attachments: 0
Username: [removed for privacy]

EndNote Web Sync Library
Serial Number: Unable to contact EndNote Web.
Records: Unknown
Groups: Unknown
Attachments: Unknown

Sync Limits
Number of Records: Unknown
Number of Groups: Unknown
Number of Attachments: Unknown

I just shelled out some serious money for this program, and I really hope it was not an expensive mistake. So far I only have a handful of references in my desktop and web accounts, so library size shouldn’t be an issue.

I am running Mountain Lion (MacOS 10.8) and EndNote X6.

Any help?