Remote Access Issues: both for remote login and ftp

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Everything was working fine for a year and all of a sudden the remote login to the Diskstation stopped working. I decided to upgrade to DSM4.0 to see if that would solve the issue, but it remains the same.

My configuration today is:

DSM4.0 (latest drop)
Static IP address xxx.xxx.xxx.104 in the Diskstation outside of the DHCP range of the router
Ports 21 and 5000 forwarded in the router to xxx.xxx.xxx.104
DDNS registered in both No-IP and Synology. The behaviour is the same using both domains or the external IP address of the router.

Description of the problem.
When I try to remotely log in the Diskstation either using the external IP address of the router or either one of the DDNS domains, there seems to be a connection to the Disktstation, (it appears "Synology Diskstation" in the tab of the browser), but the browser keeps on trying to retrieve info from the Diskstation without success. The DSM starting page never shows up.

When I use an FTP client, I can log in the Diststation and see the list of directories where the ftp user have access to. I can see the files in the directories, but when I try to download of upload something it just does not work. There is no traffic flow.

The Download Station is indeed able to download/upload traffic from the Diskstation from/to the Internet without any problem.

I would really appreciate any hint of what can be going on.