Why do I not get a console with systemd?

I'm trying to get a login prompt on an embedded system using Debian and systemd. The system is connected via a physical serial console to a host PC. If I start the kernel with the cmdline parameters console=ttyS5,115200 console=tty1 systemd.journald.forward_to_console=1 I do see all the kernel messages from /dev/kmsg and systemd logs, but I never see a login prompt. If I omit console=tty1 the kernel log is still beeing printed, but the systemd logs appear with a 30 seconds timeout for each line. What's the problem here? The same root file system works just fine for a similar embedded system.