HELP! Endnote X9.3.1 on Mac reporting "A 'sync' error has occured(200)"

I encountered this problem months ago, I referred to

which says it is because server error, so I waited and it went ok after a day or two.

I encountered the same problem weeks ago again, I thought it was the server error again and would be ok after a few days, BUT IT DOES NOT this time… I found the error message is still there today, then I update endnote from X9.2 to X9.3.1, but the error is still there. 

I can use endnote on Windows and iPad smoothly, but there is the weird sync error on Mac…I cannot sync the library, nor can I try to sign in again in the software, it seems every time I want to connect to the server, “A ‘sync’ error has occured(200)” appears.

PLEASE HELP…TwT…I will be so grateful if you could help me out TUT

Here is the Sync status:


Sync Details
Sync was last run: Wednesday, September 18, 2019 at 00:34:03 AM +0800
Sync Status: Unknown
Error Code: Client.Error.Unknown
Error Message: A “Sync” error has occurred (200).

Local Sync Library
Library Name: Mac EndNote Library-Converted.enl
References: 210
Groups: 10
Group Sets: 5
Attachments: 228

Online Sync Library
Serial Number: Unable to contact the online service.
References: Unknown
Groups: Unknown
Group Sets: Unknown
Attachments: Unknown

Sync Limits
Number of References: Unknown
Number of Groups: Unknown
Number of Group Sets: Unknown
Attachment Storage: Unknown
Permission: Unknown


The snapshot of error message is in the attachment, and I am more than willing to provide other useful information.