I have started to get this problem on one of our test systems here.
The user can logon to the server using direct server entry in logon pad. But while he tries to logon using the logon group configured in his logon pad, he gets error as partner 127.0.0.1:sapdp00 not reached.
I have checked in the host file entry that this IP is configured as Localhost only. also the actual IP address of the server is also mentioned in the host file.
When checked in the message server status area, I found that it is showing IP address of message server as 127.0.0.1
If I manually change the IP there, to the actual IP address and save it; the problem gets solved. But when I come out from smlg and again run the tcode and go to messge server area, the changes that I have done get reverted back.
Please let me know how to resolve this issue permanentely.
( message server trace shows that it has correctly picked up the actual IP address)