After we contacted informix support group about this issue they told us
that maybe the problem is
caused the time in the server is changed to some point in the past
which was older than the time
that the queries were prepared.

I've tested it again with some simple 4gl program and they were
correct. So I believe this is the
reason why this error was produced on the server.

I hope someone who has the same issue will find this post helpful.

Thanks again for all your replies.