Virtual Server 2005: The virtual machine could not be started. An unexpected error occurred.

I routinely use Microsoft Virtual Server 2005 R2 SP1 to run a handful of virtual machines for development and testing purposes. One of these VMs is set to run automatically at system boot. Every once in a while, it refuses to start up.

Trying to start it manually through the built-in status manager results in the spectacularly unhelpful and rather redundant message:

The following error occurred:
The virtual machine could not be started. The virtual machine could not be started. An unexpected error occurred.

Many sources recommend deleting the virtual machine and recreating it. This is rather cumbersome, especially if the VM settings deviate significantly from the defaults. There is an easier solution.

For some reason, Virtual Server messes up its saved copy of the user credentials used to start the virtual machine. To fix it, simply open the web interface through Internet Explorer (sadly, that interface seems to work only on IE). In the "Virtual Machines" section on the left hand menu, select the appropriate machine from the "Configure" option. Edit the VM's general properties. Re-enter just the password in the appropriate field under "Run virtual machine under the following user account:" and click OK. Then start the VM.