Are you using pidgin or other client for chat? Disable the idle function.

[Montoya]

If you use pidgin or similar client for RSI live chat, be sure to disable the idle function:

It is activated by default, but RSI live chat does not have such a feature which results in 403 errors.

 

[Shar Treuse]

I always do, so nobody knows when I am AFK unless I deliberately mark myself as such.

 

[Egriz]

Good call! I did this a while back, but since re-installing, I forgot to change this setting.