We too faced same issue earlier, but after logging in using Admin account it worked fine. Make sure you are trying to export using Admin account or try refreshing your session (logout and log in again).
Make sure you set the Cloud service and select the right Target workspace in the page properties of the immediate root folder of the Experience Fragment, You might leave the externalizer URL blank its optional
and what version of AEM and service pack are you on? we faced the same issue before