Invalid server, huh? That's a bummer. It's not something that the Director is doing; it means the bot had a problem contacting the game server. Some common causes of this are intermittent connectivity problems (you don't run your bots on a computer connected wirelessly, do you??) or software firewalls such as McAfee, Symantec or about a dozen others or random affectations from the Internet boogymen.
What can you do about it? Make sure you are not connecting via wireless. Wireless has periods of time where it just disconnects and reconnects. Sometimes you notice, sometimes you don't. Sometimes the bots notice, sometimes they just continue on as if nothing happened. Whatever happens, though, you're running the odds. You'll have a bunch of bots reconncting all at once and get an IP ban or you'll hit the one point where you're restarting a bot during that period and you'll get "invalid server".
Software firewalls are the most common. Make sure the bot's executable is in the whitelist. Each time you upgrade, make sure the new version is in the whitelist. I don't know how many times people have told me they whitelisted their bot but they're still getting this error. I ask "did you upgrade the bot recently" and get "yes" for an answer. The new upgrade is a new program. It needs to be whitelisted.