We're running a mix of linux and Windows ZCM servers, all upgraded to ZCM 10.3.1
I am currently on holiday and havent been able to do any deep debugs without showing my face in the office (and get stuck there ;-) ), but this is what I've been able to uncover is the issue:

The Windows servers suddently stopped processing the inventory sent in via clients. The clients say they have been sent in, but nothing happens in ZCC. If I change the closest server rule from the Windows server to the SLES server, it will however work next time inventory is submitted.

If I force a inventory scan of the windows server itself, it does not update itself. If I from the Windows server ask the client for inventory, it updates when the inventory server is set to the SLES server.

Any ideas what this might cause? It will greatly affect our setup, I am to setup 2 new boxes next week with ZCM as primary servers, and this will affect if I should use SLES or W2k8 R2 as the base OS.

Details about the setup:
1 x SLES 11 SP0 x64 with ZCM 10.3.1 - fully patched. This is also the dedicated PXE server for imaging operations. Originally not intended to be anything else.
2 x Windows Server 2008 R2 with ZCM 10.3.1 - fully patched. Set in different firewall zones. As far as I could see when testing, both servers behave in this way. One of them is in the same firewall zone as the SLES.
~40 clients, starting the second week in January, it will grow to around 200 and by the end of May, 3500.

Merry Christmas to all :)
May you have fewer issues by using Novell(R)'s products in the next year :)