I think this error may be unique to MSN protocol. And the plugin just isn't handling it gracefully.

When using pidgin on my desktop with MSN I would get the same error if I logged in simultaneously from my laptop. It seems like MSN would log out other simultaneous sessions (i don't know if this is true).
- However Pidgin would display error text if this happened, and offer to reconnect. Which interestingly would make the other computer's MSN connection drop out with an error too after the 1st computer stole it back!

Is there a bug report somewhere? It may be eminently fixable if it detects this and offers to reconnect. Its probably just related to breaking the session uncleanly and being unable to re-establish it.