when enabling file vault, keyboard stops responding (legacy Mavericks OS X 10.9.5)

I have a dual boot on my Mac for developer reasons. The other boot partition contains a legacy OS X (10.9.5).

I want to enable File Vault for the legacy boot partition (10.9.5). Everything works as expected until I get to the "press Restart" button. After pressing that, it asks for admin and user passwords. I am unable to type the passwords at that point because the keyboard stops responding. I've tried different keyboards (USB and Bluetooth).

After clicking "Cancel", the keyboard continues to work again. I've tried several times with different keyboards, always the same pattern happens.