In Security tab, execute "Restore Secure Boot to Factory Default", then reboot once more into UEFI/BIOS with F2. Make sure a security management password is set, so to enable several of the functions, and that Secure Boot is enabled (initially, for same reason) in the Boot tab. Enter UEFI (BIOS) with F2 once Acer appears. Install Ubuntu (it worked with 18.04 and 20.04) from a USB drive. One therefore needs to fiddle in the UEFI (BIOS) configuration to make an Ubuntu-only installation bootable: But after installing Ubuntu and formatting the HDD in the process, an error appeared at boot-up: "Default Boot Device Missing or Boot Failed", or "No Bootable Device", probably once the UEFI/BIOS has been cleared. 
I first had dual-boot for a couple of years, but then wanted to delete Windows and recover that partition. Further to Piyush, this is also how I solved this problem on my Acer Aspire e5 575.
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