Problems with a Speedtouch 585 v6 router and a NAS

My problem is that my computers and a NAS work fine with one router, while I have problems when I use another router with the same equpiment.

At home, I have an ADSL connection with a Speedtouch 585 v6 router. My WLAN consists of a desktop with Windows 7 Professional SP 1, a notebook with Windows 7 Home Premium and two laptops with Windows XP SP 3.

I have just bought a NAS (D-Link DNS-320) which I have connected to the Speedtouch router. My XP computers get full access to the NAS and can open, add and delete files of all types. In short, no problems and the NAS works as it should with these computers.

The problem is with my Windows 7 computers. They can see the volumes on the NAS, but cannot open them. Sometimes, I can copy and delete files, but most of the time Windows Explorer hangs and has to be re-started.

I also have a mobile router Huawei B260a, which I use in our country house. I have tested to connect the computers at home to this router. The NAS works without any problems with all computers, also the Windows 7 ones, and they get full access to the NAS volumes.

So, obviously, the problem is with the Speedtouch router. Should I open some ports, but I am not sure which ones and how to do this. But since the Windows XP computers can access the NAS, perhaps it is not a port problem.

The Speedtouch has four Ethernet ports and it should not matter which port I use to connect the NAS. The wireless Huawei router has only one Ethernet port, so I never had to guess which one I should use.

Has anyone any ideas how to solve this problem?