error 25588 the application process cannot connect to online server

Dear friends,

I encountered the above error when more than 32 users accessing the application. My onconfig.abc file has the following parameters:

DBSERVERNAME abc
DBSERVERALIASES abc_tcp
NETTYPE ipcshm,1,64,CPU
NETTYPE tlitcp,1,64,CPU
MULTIPROCESSOR 1
NUMCPUVPS 1
SINGLE_CPU_VP 1

I am running Informix application on:
- SUN Solaris 5.6 with 2xCPU, 1DB RAM
- Informix IDS: 7.24 UC6
- Informix 4GL: 7.20 UD2
- Currently slightly more than 32 concurrent users

I have read informix manual in order to find out if the above parameters had caused the above error, but not able to. Below are my questions:
1) Is the error -25577 caused by above parameters? If not, what causes it?
2) Do you recommend to increase NUMCPUVPS to 2 since I have 2 processors? Does it improve performance by doing so?
3) Can I have two different protocols (ipcshm & tlitcp) defined as same VP_class (CPU) but each protocol have 1 poll_threads, in fact, there is only one CPU VP?
4) What are the suggested parameters for best performance?

Thanks.

Best Regards