André,
Thanks for your reply. I indeed use a dedicated iSCSI network. The NAS is configured to have an ACL on the LUNs based on IQN. I doublechecked the IQN name and am 100% sure it is correct. Also no other things are changed, only the upgrade of ESXi 5.1 to the new update1 version. On all other machines (in this cluster and other vSphere datacenters) there is no issue.
Regards,
Stephan