Problem: a endnote sync error (200)


I tried many things but since I also could not find any more information online, I decided to give it a try by posting it here.

I am currently working with EndNote X9 (licensed) on a Mac (10.14) and I wanna use EndNote Web as a cloud to have my whole library accessible on the EndNote ipad App.

But it doesnt wor!  

I get this: A “Sync” error has occurred (200).

Sync Details as follow:
Sync was last run: Never
Sync Status: There are changes that need to be sent.
Error Code: Client.Error.Unknown
Error Message: A “Sync” error has occurred (200).

Local Sync Library
Library Name: My EndNote Library-***.enl
References: 190
Groups: 27
Group Sets: 5
Attachments: 43
Username: /

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

I don’t find anything online and I don’t understand why it worked before and now just stopped? I also don’t find any explanation to the error code.

Would be very grateful if anyone has any ideas or suggestions!