In Windows, if you are getting "Client.Error.Unknown" error message, and "EndNote could not connect to the online sync service. This may be due to a temporary Internet or Server error. Please try again."
This can often be due to TLS 1.x settings not being turned on. To turn it on:
Windows/Start > Control Panel > Internet Options > Advanced
Here you will want to look for "Use TLS 1.0" (and all higher numbers, like TLS 1.1 or 1.2 if available). If that option is unchecked, then check it and then try using Sync again.
If that doesn’t do the trick, it’s also possible that the library may be damaged, or being stored in an unsafe location that is rendering the collection too unstable to Synchronize properly.