Actually I found this problem sometime back. It was because certain range of Singnet IP is not able to reach the forum. Not only this forum, but other forums hosted on the same provider. I've contacted Singnet helpdesk, but they were not very helpful on this.
Just a tip: if you are having issues accessing the forum when you are connected, just go to
http://www.whatismyip.com. If you IP is of the form 121.x.x.x, you will have problem logging in. To solve the problem, either restart your connection a few times, till you got a 219.x.x.x or 220.x.x.x IP, or you can go to Tools --> Internet Options --> Connections --> LAN Settings, and configure proxy at proxy.singnet.com.sg, at port 8080. That solves my problem when my IP is not right. :)
It is not a browser issue, but a network related issue. :)
Seems like a transparent proxy issue.