Thanks for the advice from Mladen Pejaković. This did not work!
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Restart (ctrl+alt+del)
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Wait for the system to ask you to insert your Librem Key, press ENTER (do not insert the Key yet)
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You will get into PureBoot menu, NOW insert BOTH your Librem Key and your Librem Vault (gold USB drive)
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Select Options → Update checksums and sign all files in /boot
But when I braved your other selection below, it worked. I just let it take its time, and finally I got into the system. Thank you.
P. S. You can ALWAYS boot to your system WITHOUT Librem Key: from the PureBoot menu select Options → Boot options → Ignore tamper and boot → Continue → (press ENTER on whatever first option is selected) → on red screen prompt type your disk encryption password and press ENTER to continue