My colleagues Laptop Dell Inspiron 15-3567 (System Bios: 2.9.0, ePSA: Build 4304.14 UEFI ROM) with OS Windows 10, boot failed with error "Operating System Loader Signature Loader failed signature verification. WARNING: The file may have been tempered with! All bootable devices failed Secure Boot verification." (Note that I am writing this down so that this post shows up in Google Search). The steps listed below worked for me:
1. On Startup, press F12, and on the options, Select BIOS Setup.
2. Reset Bios to default settings (in my case I did Factory Reset).
3. Restart Laptop, go to BIOS setup again.
4. Firstly, Disable Secure Boot Option.
5. Then, Select Boot Sequence menu.
6. Manually ADD additional line with the name "Windows Boot Manager".
7. Select file /EFI/BOOT/BOOTX64.EFI for the entry created in step 7.
8. Exit and Restart. If the Laptop does not boot then uncheck default UEFI boot option in Boot Sequence menu from BIOS, so that the Laptop boots from the boot entry created above.