Basically ESXi 6.0 with vSphere runs on Xserves.
After some time, i found out that it was not the server crashing.
It was the network connection that went down.
I knew this because a) the server was not dead, no PSOD or anything, and on my secondary network interface, the server was still pingable.
So, what did we do:
The Xserve 3.1 comes with these NICs installed:
Intel Corporation 82574L Gigabit Network Connection
So what we did is exchange the PCI NIC to a
Intel Corporation I350 Gigabit Network Connection
Soon after that, VMware released ESXi 6.0U1.
And what do I read in the Release Notes:
An ESXi host might lose connectivity and e1000e virtual NIC might get reset
packets completion seems stuck, issuing reset
This issue is resolved in this release.