ESET Antivirus has updated their program and introduced a feature named Host-based Intrusion Prevention System (HIPS).  This allows the system to lock down any program that may be manipulating files on the operating system.  HIPS views PGP Desktop on an encrypted disk as "harmful" and does not allow the program to launch.

You receive errors stating that PGP Desktop is not installed and it cannot find the required files.

You only see these errors after the disk is fully encrypted, not before or during the process.

The workaround currently is to disable the HIPS function within ESET.  This allows the PGP Desktop client to be launched again without issues.

The disk will remain encrypted and you can then follow the normal command line options for decrypting If you are still having issues.