This was due to our App Stack name being Too long.
We upgraded an existing app stack.. ( which the name length was fine ) .. however this now created a VMDK for the app stack that was too long.
There was No warning.. the symptom was the app stack just doesnt load.. you get a virtualization error when loading the instant clone w/ the app stack applied.
Thanks to crazy awesome vmware support they were able to track it down.
This will appear in the app stack manager log :
"The filename is too long (msg.disklib.NAMETOOLONG)"
also for some reason i didnt 'catch' that the app vol manager logs were \\app vol server\log .. that's where its location.
not sure if client said anything about it as we only checked the manager log.. and stopped - as we found the error.
It appears Spaces take up 4 characters ?.. is that common for linux ?.. sorry im a windows guy. So a space in an app stack gets converted to "!20!" in the name of the appstck VMDK.
We will just stop using spaces and just use dashes..and/or make smaller names.