After a reboot of a server, it can happen that it is not possible to take a quiesced snapshot.It happened after an update to 6U1 from 6.0. Before the update there were no problems.
We did the following to troubleshoot the problem.
* Reinstall VMWare Tools
* Check VSS Providers for errors (no errors)
What we found out was that it can happen after a reboot of the server and only if the service "VMware Snapshot Provider" is running (service is set to manual). To correct it we need to stop "VMware Snapshot Provider" and "VMware Tools". If all is stable, ie no running services, which should not run, like "Volume Shadow Copy" we start the service "VMware Tools". After this we can take a quiesced snapshot again.
While this ain't a huge problem, it is annoying at best. Any one have a solution or have the same problem.