Later I found that I had disabled „Boot from side USB“ in UEFI advanced security settings. I worked around that using Advanced Startup Options: The current standard Microsoft tools allow to create NTFS only. USB sticks for booting with UEFI must be formatted as FAT32. Via holding volume-up key while releasing the power button.
(select GTP partition scheme for UEFI, FAT32, the Windows.iso downloaded). Via Windows 10 using the media creation tool. I used the following steps to do a clean install: Starting Win 10 setup from within Windows does not allow to remove unneeded partitions. For a really clean Win 10 install one must boot from installation media.
