Just had a look at this by comparing the list of kernel modules loaded just before cryptsetup waits for passphrase entry at boot, with modules=most and =dep.
By that I found out, that with =most the module psmouse is loaded while it is not with =dep. If I add psmouse to /etc/initramfs-tools/modules, then boot also works again with with =dep. Strange... because no mouse or touchpad is involved... but who knows what's going on there... So at least this is needed on the Fujitsu Lifebook U757... maybe it should be generally added.
