How to Sync my Endote desktop Library with my online Library

I am trying to sync my desktop library with the online one but I have not been successful . the error message is:

Sync Details
Sync was last run: Never
Sync Status: There are changes that need to be sent.
Error Code: Client.Error.Unknown
Error Message: EndNote “Sync” could not connect to EndNote Web.

Local Sync Library
Library Name: My EndNote Library.enl
Records: 734
Groups: 3
Attachments: 1224

Online Sync Library
Username: mixedcrown221181
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

How do I go about resolving this problem?

Many Thanks in advance.