Hi There! I think I have the same problem. Did you find the solution?
Sync Details
Sync was last run: Wednesday, November 07, 2018 at 10:42:39 AM +0100
Sync Status: There are changes that need to be received.
Error Code: Server.AuthenticationToken.invalidUserCredentials
Error Message: The username/password specified is not valid.
Local Sync Library
Library Name: My EndNote Library.enl
References: 1152
Groups: 0
Group Sets: 1
Attachments: 2
Username: a.e.de_wit@lumc.nl
Online Sync Library
Serial Number: 3091856020 (Premium until December 29, 2019)
References: 1152
Groups: 0
Group Sets: 1
Attachments: 2 (29.73 MB)
Sync Limits
Number of References: 1000000 (998848 can still be added)
Number of Groups: 5000 (5000 can still be added)
Number of Group Sets: 5000 (4999 can still be added)
Attachment Storage: Unlimited
Regards!