ACER

PROBLEM SOLVED! It turned out the problem was in the bios settings. I switched "secure boot" on which unlocked few more options. One of them was "Select an UEFI file as trusted for executing" that's in the "security" tab. After that it showed me HDD0, I hitted enter and "EFI" showed up. After another ENTER it showed 3 folders "ubuntu", "microsoft" and "boot". The ubuntu folder contained "shimx64.efi", "grubx64.efi" and "MokManager.efi" I'm not very familiar what all of them were, but I chosed "shimx64.efi" which was then added as "trusted for executing". I went back to the boot tab and found "EFI boot0: ubuntu" or something like it. That's because I named the "trusted" UEFI file "ubuntu". I moved it up to first priority and everything worked like a charm. Thanks for the advices, they made me search for my own mistakes in the bios configuration.