How to fix: 403 Forbidden: The server understood the request, but is refusing to fulfill it.

Getting this message? Me, too. A search on Google yields 25,000 results. The only search results that seem to explain it are these:

1) We were having some trouble with database replication lag.

2) Why Does 403 Forbidden Happen? There’s actually a number of reasons why these annoying little buggers happen, but the most common is caused when new programs are installed over all old ones without the old ones being completely uninstalled 100% first, causing registry pile ups. Another reason, is damage that’s been done by malware/spyware/adware programs, attaching themselves to your PC and wrecking havoc, deleting crucial files from your PC.

3) work around in firefox: click tools -> options -> privacy
click show cookies button, open twitter.com folder, delete _twitter_session cookie.

4) There are several reasons. On the server side, that could mean:
• Congested server and/or network and/or datapaths
• Errors in accessing database, files, network and other resources that are badly handled (meaning: not sufficient, even not at all).

On the client side, it could be that
• Your browser doesn’t behave as it should
• Your connection is congested

I don’t want to scare you, but I need to ask this since you appear to ask from a Windows-based machine: Have you checked your system recently on malware (Rootkits, Virus, trojans), and do you run a virus-scanner that is up-to-date?

The reason is that these ‘products’ can cause a lot of traffic without you knowing it. It could lead your ISP and certain sites to block you – causing more or less permanent 403 pages.

Ruling out your PC as cause of trouble: Is you wireless connection protected (access key, encryption)? It’s another possible source of abuse.

You may well be using a proxy server and a firewall, whether you realize it or not. Various of these capabilities can exist in your WiFi router, and the ISP can configure these into its network.

This could be malware, or it might not. It’s also possible that there are parental controls, a routing problem, or a device hardware or software or configuration error within your WiFi device.

Borrow a Mac laptop or a Linux box from somebody and see if that works; Mac and Linux boxes are not usually vulnerable to typical Windows pestilence.

Any of these borrowed boxes can be used to troubleshoot the configuration; to determine if there’s a problem with your router, your network, or elsewhere.

Partial source: the ITRC Microsoft forum
http://forums1.itrc.hp.com/service/forums/familyhome.do?familyId=116

Another informative article: How To Fix The HTTP 403 Forbidden Error at eHow.com