I have set UEFI/Legacy Mode to Both and also disabled quick boot mode without any success to boot from any USB and I have several and tried all ports as well. I am planning on installing Ubuntu Mate on the SSD which currently has Windows 10 installed and remove Windows completely. Would I have to use UEFI or Legacy then? Any other ideas how I can get this machine to boot from USB?
UPDATE: I just ran the Lenovo Update Utility and flashed the BIOS to 8DET70WW (1.40), dated 2015-05-14. Even with this update there is no visible option to deactivate Secure Boot in the security sub menu in the BIOS. I initially thought that the BIOS has a hidden Secure Boot function but won't let me deactivate it and had hoped that a BIOS update would solve this problem. The BIOS update did not have any effect on the USB booting situation.
SOLUTION: It seems to me that the X220 supports UEFI but not Secure Boot. I created another bootable flash drive, this time with an old HP and guess what, it booted without any problems. For some strange reason, the X220 won't accept any of the newer SanDisk USB drives. Good to know! 
Thank you @wolfman for your continuous help, it prevented me from throwing the X220 out the window and it is indeed a prime example of what you very often tell people on this forum: that we sometimes overlook a very simple solution. @ouroumov Thanks for your help as well. Through this little detour I successfully updated a BIOS for the second time and learnt a lot!
