I disconnected all the external usb devices, manually deactivated the background image, used grub-mkconfig and update-grub. I should tell you that the behaviour changed recently: First, the menu is shown and the selection of the first menu entry leads to the long waiting. This is with and without background image. That lead me to the question if there's something wrong with the kernel.
I think that's it dmesg: Due to kernel problems I don't yet understand, My hdd is started really late and so it seemed like grub froze.
So, the problem changed from grub to kernel. Let's see what the seardh machines say to this.