Since I've encoutered that bug, I have changed two lines in /etc/default/grub:
#GRUB_GFXMODE=640x480 -> GRUB_GFXMODE=1280x800
GRUB_GFXPAYLOAD_LINUX=1280x800 -> GRUB_GFXPAYLOAD_LINUX=keep
Now the typed text isn't echoed on the side of the screen anymore, but I still can't enter my password.
Since I've encoutered that bug, I have changed two lines in /etc/default/grub: 640x480 -> GRUB_GFXMODE= 1280x800 _LINUX= 1280x800 -> GRUB_GFXPAYLOAD _LINUX= keep
#GRUB_GFXMODE=
GRUB_GFXPAYLOAD
Now the typed text isn't echoed on the side of the screen anymore, but I still can't enter my password.