I encountered the same problem with my LinkStation Pro Quad. Here are my settings:
WebAccess Service: Enabled
HTTPS/SSL Encryption: Disabled
Use BuffaloNAS.com: Enabled
BuffaloNAS.com Name: xxxxxxxxx
BuffaloNAS.com Key: xxxxxx
DNS Hostname: -
Auto-Configure Firewall (UPnP): Disabled
External Port: NAS Internal Port: 9000
Exclusive session: Disabled
Sessionexpiration time (minute): 30
WebAccess URL: http://buffalonas.com/xxxxxxxxx/
If i use my external IP address, it works (ie: 220.xxx.xxx.xxx:9000). However if i use the buffalonas.com webaccess link, it resolves to a wrong IP address, and therefore cannot connect.
How to solve this problem?