Many customers are hitting a new bug in ESX preventing them from powering on virtual machines or VMotion them.  There is a published knowledge base article on this which reads>>
Unable to Power On virtual machine with “A General System error occurred: Internal error”
A virtual machine fails to power on, fails to leave suspend mode, or [...]

With the new Virtualcenter 2.5 Update 2 comes some tighter restrictions on HA network configurations.  This error message commonly occurs when the nodes in a cluster contain incompatible networks. An attempt to reconfigure clusters might fail now if they do not have identical networks.
Some customer are reporting the message:

HA agent on <esxhostname> in [...]

As soon as Update 2 of Virtualcenter 2.5 hits the streets we started getting calls into support lines here at VMware about HA breaking.
Common reports include:
“insufficient resources to satisfy HA failover level on cluster” and “unable to contact a primary HA agent in cluster”
We think we know what is causing this, so I’m sharing this [...]