Q: suddenly no connect to database

Hello out there, I've installed IDS Workgroup Edition V 7.30.TC7 on NT 4.0SP4 several times. But now I've a problem with one server: The user starts an application on a client PC with Informix-Connect V7.20 and works. Sometimes the user gets the error message "-349 Database not selected yet", although there's no "close database" statement in the application. The user must restart the application and can continue his work, until the next -349 error. The same application runs fine with other NT servers, so I cannot believe that it has something to do with the application itself. I've made a protocol of the network trafic for one day, but I cannot see anything. Has anybody an idea, what's going on here? Every hint is welcome! Thanks in advance Dirk Scheer