Okay again as explained in the following thread this is caused by my PC flipping between connecting to different point of the network.
http://forums.dlink.com/index.php?topic=72778.0
For the benefit of the others who may have been confused with what I described, the reason I lost access to the Web UI only after 3c was because after 1a my PC was disconnected from one end of the network and reconnected from another point of the network back to the switch, which still have Management VLAN on VLAN 1 so the Web UI was still accessible. It was only after I have changed the Management VLAN to VLAN 100 did I lose access to the Web UI also via this other path.
1) VLAN -> 802.1Q VLAN
a) Click on VID 1, change port 6 from Untagged to Not Member.
This takes Port 6 out of VLAN 1.
b) Click Add, enter VID = 100, VLAN Name = Mgmt, change port 6 from Not Member to Untagged.
This puts Port 6 into VLAN 100.
At this point I'm still not locked out because I think 3a below is Disabled so the switch doesn't care which VLAN you are on yet.
2) 802.1Q VLAN PVID
Nothing needs to be done here but Port 6 is now assigned with PVID 100.
3) 802.1Q Management VLAN
a) Change Management VLAN from Disabled to Enabled.
b) Change VID from 1 to 100, the VLAN Name is changed from default to Mgmt automatically.
c) Hit Apply.
As for below this is absolutely a good advise. Coming form a CLI background such as Cisco, it is easy to be able to set the native VLAN of the port you are connected to the switch with just one command. On the Dlink switches via the Web UI, it's a two step process. First you go to the VLAN 1 page and check the port to be Not a Member of VLAN 1. Then you go to the new Management VLAN page and check the port to be Untagged. After the first step before you get the chance to do the second, you are already locked out. A good lesson learned here with switches like the Dlinks that need to be configured this way.
The only bit I'm not following is why the need to first connect on a port different than Port 24 before putting Port 24 to the Management VLAN, and then connect the laptop back to Port 24 after getting locked out.
This is because I think it's not a good idea to manage the port your admin PC is connected to. And I'm really astonished that you could still do any configuration beyond step 1a, because my understanding so far was, that per default the switch's management interface is only reachable via VLAN 1! But maybe this is due to a time gap between the point in time when you change a configuration and the point in time, when this change comes effective.
