Hi Everyone,
Most of the times we face this error message in the Web dispatcher trace file (dev_webdisp) stating that connection to message server is not possible.
There are different reasons for this to occur. Most of the times the issue is occurring because of wrong configuration of the message server port in the Web Dispatcher port.
We can configure the parameter related to message server port HTTP(S) in the Web Dispatcher host profile. The port on which the message server receives HTTP requests
The common entries occurring the trace file looks like:
*** ERROR => Connection request from (-1/65535/0) to host: <message server hostname>, service: <message server port> failed (NIECONN_REFUSED) {001d7388} [icxxconn.c 2712]
*** ERROR => IcmConnClientRqCreate() failed (rc=-8) [icrxx.c 6910]
*** ERROR => Could not connect to SAP Message Server at <message server host> URL=/msgserver/text/logon?version=1.2 [icrxx.c 3870]
*** ERROR => rc=-1, HTTP response code: 0 [icrxx.c 3871]
These entries show us that web dispatcher is not able to connect to message server on the http port.
To check this we have one solution to do.
We can check the connectivity of the backend message server's http port by using simple telnet command.
How to check the connectivity using telnet ?
You can check this by using
telnet <message server hostname> <message server port>
You can find the message server http port in dev_ms trace file
where entries usually would look like:
I listen to HTTP port ****
You should be able to successfully test telnet command. The result would be something like :
telnet <msg server hostname> <msg server port>
trying to connect ..
connected successfully..
Hope this information helps
Cheers 
