Acer Aspire V3 does not recognize my EFI partition

I finally solved the issue, Kubuntu is able to boot with the default UEFI firmware settings (no legacy mode, secure boot enabled). EFI/ubuntu/shimx64.efi needs to be selected once in the UEFI (press F2 during boot, set a supervisor password, then got to Select an UEFI file as trusted for executing and pick EFI/ubuntu/shimx64.efi). The password can be removed afterwards. Here some background information: SHIM (shimx64.efi) is a (Microsoft) signed binary used to call GRUB (grubx64.efi); GRUB then loads the Linux kernel. By the way when I boot in legacy mode Kubuntu is unable to shut down correctly and crashes every time I try to change the mouse settings; this is solved after switching to UEFI mode.