Something very like this appears from prior discussions to have been a problem for multiple people under circumstances that I could not identify.

I am using macOS 10.14.6 on a late 2015 27-inch iMac Retina 5K  – –  also known as iMac17,1.

I have four virtual machines:  Windows 8.1, Windows 10, macOS 10.13, and macOS 10.14.

All were working with Fusion 11.5.5, all are identically broken now that I have “upgraded” to 11.5.6.

When I try to start the virtual machine the following error dialogs are displayed:

1.  Could not open /dev/vmmon:  Broken pipe. (In a free-standing box on the same external monitor as the virtual machine library window.)

2.  Failed to initialize monitor device.  (In a drop-down box from the title-bar of the virtual machine’s window on the iMac itself.)

3.  Transport (VMDB) error -14: Pipe connection has been broken.  (In a drop-down box from the title-bar of the virtual machine’s window on the iMac itself.)

The TeamIdentifier hack discussed in the other thread did not make a difference.

Re-installing from a freshly downloaded copy of the installer .dmg did not make a difference.  It did NOT require me to re-enter the license key, so obviously it did not start by removing everything.  It gave me no choices about what to install or where.