You need to check the firewall installed on your computer. Go to Control Panel > System and Security > Windows Firewall. You can either add the software to Exclusion list or simply turn it off then try accessing the drive again. I have encountered the same error with other cloud storage solutions as well and this fixed it. Additionally, check the login details in the Asus WebStorage.
If you still encounter issues, contact the Customer Care that's always available for users who access the service: https://service.asuswebstorage.com/