Informix 5.0, 5.01 and I-Net PC

Hi, all ! I have got some trouble with Informix Online 5.01 on SCO when communicating with I-Net PC 5.01. In our network we have some PC's with SCO Unix and Informix 5.0 and 5.01 (one PC wasn't upgraded). All connections via I-Star between 5.01 and 5.0 Informix hosts works fine, but if I wish to connect to Informix 5.01 with I-Net PC 5.01, it reports that error 457 (SOL engine terminated unexpectedly) has occured. Conntections with I-Net to Informix 5.01 works, connections with I-Net 4.?? to any Informix works too, but ... I can to connect to 5.01 databases via 5.0 server (it works !), but it seems a bit inconvenient to me. All configuration files are the same on each host in network. Can anyone tell me, what's the matter ? Maybe I must upgrade to 6.? WBR, Vadim J. Nikitin