HP DL360 G6 "Non-Compliant" with ESXi 5.1

Hi,

I've two DL360 G6 running with ESXi 5.0U1 and wanted to upgrade them to 5.1.

I've downloaded die HP-ESXi-5.1.0-standard-iso and added it to Update Manager, but when I check for an upgrade I get the error that the servers are "Non-Compliant".

From the VMware Compatibility Guide the DL360 G6 should be supported for ESXi 5.1.

Is there any way to find out why it's Non-Compliant?