I've an older X58 chipset based desktop which is giving 1803 update issue. After pouring through lot of forums finally found the issue with a BIOS setting called "Intel Virtualization Tech for Directed I/O" ... Sometimes named as "VT-d" ... It was enabled in my system for Hyper-V. After disabling that BIOS setting the install succeeded.
Caveat : Hyper-V won't work after you disable VT-d and you can't enable them post the upgrade as that will result in system freeze during boot.
Hope it helps some folks