Today I successfully reproduced the installation process.
OpenBSD 6.4 Machine type/ model: 7945B2G Host firmware (UEFI) 1.22, build date 2018-06-04 System settings - Processors Power C-States <Enable> Report C2 to OS <Disable> Virtualization <Disable> VT-d <Disable> Cache Data Prefetch <Enable> Data Reuse <Enable> Execute Disable <Enable> Hyperthreading <Disable> System settings - Memory Memory Speed Max Performance LV-DIMM Power <Low Power> Memory Channel Mode <Independent> Socket Interleave <NUMA> Patrol Scrub <Disable> Demand Scrub <Enable> Static Driver Impedance <Disable> Memory Refresh <1x> Thermal Mode <Performance> System settings - Devices and I/ O Ports Configure IDE mode <Compatibility mode> Interrupt Round Robin <Disable> System settings - Operating Modes Choose Operating mode <Performance Mode> System settings - Legacy Support Rehook INT 19h <Disable> Legacy Thunk Support <Enable> Boot Manager Change Boot Order <USB Storage> <CD/ DVD Rom> <Hard Disk 0> The install medium must be the first in the boot order list. You have to boot without using the Boot-Manager (F12).