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.