No login possible via serial

[Patagonicus]

After the system has booted, the last lines I get via serial are

[  OK  ] Started Getty on tty1.
[  OK  ] Reached target Login Prompts.
[  OK  ] Started User Login Management.
[  OK  ] Started DHCP Client.
[  OK  ] Reached target Network is Online.
[  OK  ] Started SSH Daemon.
[  OK  ] Reached target Multi-User System.

It doesn't seem to accept any input (which it does beforehand, during petitboot and also in the initramfs). After systemctl start getty@ttyS0 the log for that service says /dev/ttyS0: not a tty, which I'm not sure how to interpret.

[considerate]

@Patagonicus Sorry for not getting back to you on this in such a long time. I personally only use the serial via USB and I connect to it with sudo screen -a /dev/ttyUSB0 115200.