Cannot boot from usb after upgrading to ubuntu 20.10 on macmini

Hi, I recently installed ubuntu 20.04 on macmini (2012) - it was a fresh install on an empty new disk.
The installation went good. After the install I did all the updates and upgraded to 20.10 version. Now I wanted to do some partitioning (and maybe install another distro alongside), but cannot boot from usb anymore. Within ubuntu the system sees the usb without problems. But when I want to boot into the usb after restart - it doesn't see the usb, the computer boots straight into ubuntu.

Any help?