Endnote X8 for Mac syncing errors

I’m struggling with syncing my local library to Endnote’s cloud storage. 

The login details all work through the web browser, but the sync in Endnote continues to give me the same error [below]. 

I’m using Endnote on my Mac (10.13.4) and have 8.2 (Bld 13302). 

Any help would be greatly appreciated. 

Sync Details
Sync was last run: Monday, May 28, 2018 at 19:50:27 PM +0200
Sync Status: There are changes that need to be sent.
Error Code: Server.ReferenceStore.internalError
Error Message: An error occurred while trying to upload the attachment(s). Please try again.

Local Sync Library
Library Name: Endnote Library updated.enl
References: 4633
Groups: 11
Group Sets: 2
Attachments: 8473
Username: xxx@xxx.com

Online Sync Library
Serial Number: 
References: 4633
Groups: 11
Group Sets: 2
Attachments: 7081 (4.03 GB)

Sync Limits
Number of References: 1000000 (995367 can still be added)
Number of Groups: 5000 (4989 can still be added)
Number of Group Sets: 5000 (4998 can still be added)
Attachment Storage: Unlimited

Please contact Technical Support directly

1-800-336-4474 option 4 then 1

or

https://support.clarivate.com/Endnote/s/technicalsupport?language=en_US