-25588 / -1803 after connection has been established

I have following Problem on Informix Online V7.21 UD1: Running on a Siemens RM400 Sinix V5.42 (256 Meg Memory). My Application written in ESQL/C Connects to the Database and starts a big select( 20000 Rows) . For every Row the Application selects aditional information from other tables and inserts the Results into another table. After nearly 1500 processed rows the Application gets an Error -25588 (connect over Shared Memory) or -1803 (connect over TCP / IP). Other Applications ar not affected. The Application has been running on IFX SE V6! and V7 SE for Years! Support told me to switch from 'database dbname' to 'connect to dbname@host_service', but this didn't help. 'online.log' shows nothing. onconfig and sqlhosts seem to be alright, dbaccess works. Smaller Jobs work fine too. Any Idea would be apreciated. In case of 'rtfm': which book, which page ? Any help in this matter would be very much appreciated Christian Schroeder