If your server shows with a red "X" in the browser, this means that your client is running a different version than the server. If you loaded a beta on your client, or upgraded to a new release before it was pushed out through Steam, try downgrading.
If your server shows with a red disc in the browser, then one of two conditions is true:
1. You have the server configured with equalModRequired=1 in the ./server.cfg file, and your the -mod parameters on your client are different than the server's.
2. You have add-ons loaded on your client that the server doesn't have, and the server has key verification switched on via verifySignatures=1 in the ./server.cfg file.