1.See if you are going through a proxy server (e.g. Microsoft ASA server) and the port that Advantage is using has protocol checking turned on. If so, see if this can be turned off or a rule set up to bypass this check.
2.If traffic is being routed through a proxy server, see if the firewall can be configured to bypass the proxy server for both the apps server and DB server.
3.Your client Advantage dll's are of a higher version then the Advantage Database server version. Downgrade the client's Advantage DLL's to a lower version.
4.Advantage Database is not running on the server.
5.The IP addresses in the ADS.INI point to a server that does not have ADS running.
6.The Host URL is pointing to the wrong server.
7.The MTIWEB.INI file has URL's that are pointing to the wrong server.