You may see many of the following warnings on the Microsoft Host Integration Server:

Log Name:      Application
Source:        SNA Base Service
Date:          xxxxxxxx
Event ID:      561
Task Category: None
Level:         Warning
Keywords:      Classic
User:          xxxxxxxx
Computer:      xxxxxxxx
Write to mailslot or socket failed, rc = 64


 A Win32 WriteFile() or winsock sendto() call failed. The return code is shown.


 Provide network support personnel with the event log file(s) related to SNA, and the return code included in this message. For information about SNA log files, see the "Microsoft Host Integration Server Online Books.

Microsoft Host Integration Server tries to communicate with all servers/clients in a subdomain using broadcasts messages. It does this over all protocols that are selected in the SNA Manager.
Not all of these protocols are enabled on your network, if one of them is not enabled this warning will pop up.
To fix this warning unselect the protocol that causes the warning as shown in the screenshot below:
Normally you should have enabled only TCP/IP. The warning will disappear after you have unchecked the Named Pipes.