Adding extra IP to NIC causes connectivity issues for main IP (Win Server 2012 / IIS)

I am experiencing a very odd situation:

I have set up 8 IP addresses in a single NIC (plus it's main IP) for use as a web server - and have 8 websites happily running in IIS8.

(The server in question is a Win2012 VM running in VMware vSphere)

When I come to add a 9th IP, I start experiencing connectivity issues on the server's main IP - IIS cant access the remote SQL server or Progress server (I also cant access the SQL server in SQL Server Management Studio from the server. which suggests its not an IIS related issue)

Therefore 2 of the websites throw errors stating they could not connect to their respective SQL / Progress server.

There are 100% NO IP conflicts, and I have also tried using fresh unused IP's from the pool, but I get the same result.

I didnt think there were any limits on IP addresses in Windows, but could this be VMware related?

Remove the IP and reboot, all is good again...

Day 2 - Update:

So after bashing our heads together all day yesterday and sleeping on it, I decided to add a secondary virtual NIC. Leaving the 8 working IPs on the first one, and adding the others (currently 3, but will be 8) on the secondary.

This works.

So where does this point? a VMXNET3 NIC driver issue, a VMware routing issue, a windows server issue... god knows...!

Added by request:

and IPCONFIG /ALL