Hi,
gnehme wrote:
but how do you explain the fact that i could still configure it using the vSphere (thick) client?
tricky question! 
When you move a host to a HA cluster the automatic startup/shutdown is disabled on that host. The documentation warns you that it is not wise to manually enable it afterwards as it might cause trouble in an HA event, but it was possible, for whatever strange reasons.
And here is the strange part I just noticed:
In vSphere 5.0 Update 1 it was not possible to manually enable it in the C# Client. In my 5.1 Infrastructure here it is possible again to enable it.
I really can't tell you why they brought that feature back into the C# Client. But as the C# client is probably going to die in the next release, I would think the Web client is the one that is doing it the way it is intended, disabling the ability to mess is up
Regards