troubles with transport control

Without any change in my configuration, i started to have troubles with the connection to my database (INFORMIX-SE Version 7.24.UC5/Linux). When I try to call select the db with isql client I receive following error: 25591: Transport control received an invalid connection address. I checked all sqlhosts, INFORMIXDIR, INFORMIXSERVER, /etc/hosts, /etc/services, all the settings are correct. I encountered the error once before but suddenly it started to work fine again. Very mysterious ... oliver