I did a fresh install of Ubuntu removing Windows and every data from the drive. I chose the option to encrypt my disk pre-boot.
When I boot Ubuntu out of GRUB, I get a black screen for a second or two but then the screen where I have to enter my passphrase shows up and everything works fine.
However, my laptop isn't supposed to always boot into GRUB (Ubuntu being the only OS). When I boot the machine without GRUB, all I get is a black screen that doesn't initialize even after waiting for a couple of minutes.
It is possible to press Ctrl+Alt+T on that black screen, and it yields some kind of boot console saying
Enter passphrase: **error: unexpectedly disconnected from boot status daemon cryptsetup: cryptsetup failed, bad password or options? Unlocking the disk /dev/disk/by-uuid... (sda5_crypt) Enter passphrase:
So I can now put in my passphrase into that white on black console screen and Ubuntu will boot normally (without the Ubuntu splash screen though). I furthermore noticed that I am able to enter the password "blindly" (i.e. without bringing up the console first), hit enter and Ubuntu will boot as well.
I did already try the boot-repair utility and reinstalling Ubuntu. I would very much like to see the usual "enter passphrase" screen instead of having to bring the boot console up first, but I'd also be glad when I could manage the machine to automatically boot into GRUB, wait 0-1 seconds, automatically choose Ubuntu and then get to that passphrase entry screen.
GRUB_HIDDEN_TIMEOUT like suggested in this thread didn't work for me, though. It still yields the same result as if I was booting the machine without GRUB.