Our service provider uses a sophisticated firewall in an effort to protect DU's servers from DOS attacks and other hazards. For reasons that are still unknown to us, sometimes the firewall "overreacts" and blocks access to the site for some legitimate traffic. When this happens, you get the "forbidden" error, or a "403" error. It means the firewall is blocking your attempt to access certain pages on the site.
The firewall adapts, so after a relatively short period of time -- usually no longer than an hour or two -- it recognizes that a connection attempt is legitimate and lets the traffic through. At that point, the error will go away.
If you're using a VPN, my advice would be to try turning it off when connecting to DU (VPNs often use IP addresses that raise flags with firewalls). You may also have success by connecting to upload.democraticunderground.com instead of www.democraticunderground.com. Otherwise, I'm afraid the only solution is to wait a bit for the error to clear itself.
It's annoying, but unfortunately we haven't yet been able to come up with a satisfactory solution. It's something we're still trying to fix. My apologies for the inconvenience.
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