An organization has a HA/DRS cluster containing a mixture of ESXi 4.1 and 5.5 hosts in the cluster. An ESXi 5.5 host has failed and HA attempted but failed to restart the virtual machines on another h

VMware VM hardware versions have a minimum ESXi host version requirement. A VM with hardware version 10 (introduced in vSphere 5.5) cannot be powered on by an ESXi 4.1 host, which only supports up to hardware version 7. When the only ESXi 5.5 host fails, HA attempts to restart those VMs on the remaining ESXi 4.1 hosts but fails due to this hardware version incompatibility, even with admission control disabled.

With admission control disabled, HA will still attempt VM restarts regardless of available resources; while resource exhaustion can cause power-on failures, the mixed-version cluster context points specifically to hardware version incompatibility as the cause.

HA restarts VMs by issuing a power-on operation on the target host - it does not use vMotion - so cross-version vMotion compatibility is entirely irrelevant to HA failover.

HA failover is not blocked between different ESXi versions per se; the failure condition is VM hardware version incompatibility with the target host, not a blanket restriction on cross-version HA.