I found a similar question but it has no satisfactory answers.
I have a D-LINK 2540U DSL router. It has a basic port forwarding(under DNS -> Virtual Servers) configuration in the administration panel where you specify: external port range, protocol, internal port range, server IP address and it is supposed to forward that port to that IP address.

When I first set it up for a Real VNC connection it worked fine, just as I expected. Then I added a DynDNS configuration entry in the router's 'Dynamic DNS' section and added an additional SSH (22) forwarding rule. The SSH forwarding also worked fine (now with the dynamic hostname, but I suppose it doesn't make any difference as far as SSH is concerned).

Then I removed the SSH rule and after that the VNC forwarding stopped working with the VNC client failing to connect (I have tried to connect with telnet and it also failed to connect, so it wasn't a VNC problem).
After adding a rule for port 80 it turned out it would forward on port 80 though not to the specified server IP but to the modem itself. At least it is what it looks like, because it gives me the administration panel when I connect to my external IP (both using a browser and plain telnet in which case I can see that it is mini_hhtpd sitting on the port, which is obviously the modem's administration panel).

Have anybody encountered a similar problem with port forwarding?

I have tried to do a reset through the administration panel and to restore a backup of the settings made before I started playing with port forwarding, but it didn't help.

Should I do a 'hard' reset with the button on the modem? Is it any different from the administration panel's reset (Restore default)?