Debian not recognizing my ttyS0 / console redirection not working

I have a fitlet2 (https://fit-iot.com/web/products/fitlet2/) which has a serial port. I can connect to the serial port of the fitlet2 with screen from my computer (screen /dev/ttyUSB0 115200) and access the BIOS, after I have enabled the "Serial Port Console Redirection" in the BIOS. I am also able to access the GRUB Bootloader over serial. Access over serial works in various GRUB bootloaders (debian/ubuntu installer, debian installed). But whenever the system is loaded, I can no more access the serial port which I was using. I tried to activate console redirection in GRUB for the system by editing the boot parameters according to this post https://www.cyberciti.biz/faq/howto-setup-serial-console-on-debian-linux/ . Soon I realized that my ttyS0 is somehow not working correctly. Starting to debug my physical serial interfaces I just couldn't find a way to get this to work. Setserial tells me the following: /dev/ttyS0, UART: 16550A, Port: 0x0000, IRQ: 4 /dev/ttyS1, UART: 16550A, Port: 0x0000, IRQ: 5 /dev/ttyS2, UART: 16550A, Port: 0x0000, IRQ: 7 /dev/ttyS3, UART: unknown, Port: 0x02e8, IRQ: 3 dmesg | grep tty this: [ 0.000000] Command line: BOOT_IMAGE=/vmlinuz-4.9.0-8-amd64 root=/dev/mapper/pen--01--vg-root ro console=ttyS0,115200n8 console=tty0,115200n8 [ 0.000000] Kernel command line: BOOT_IMAGE=/vmlinuz-4.9.0-8-amd64 root=/dev/mapper/pen--01--vg-root ro console=ttyS0,115200n8 console=tty0,115200n8 [ 0.000000] console [tty0] enabled [ 0.000000] console [ttyS0] enabled [ 49.308213] console [ttyS0] disabled [ 49.312635] dw-apb-uart.0: ttyS0 at MMIO 0x81424000 (irq = 4, base_baud = 115200) is a 16550A [ 49.407713] usb 1-8.2: Qualcomm USB modem converter now attached to ttyUSB0 [ 49.410489] usb 1-8.2: Qualcomm USB modem converter now attached to ttyUSB1 [ 49.411349] usb 1-8.2: Qualcomm USB modem converter now attached to ttyUSB2 [ 53.577143] console [ttyS0] enabled [ 54.744793] dw-apb-uart.1: ttyS1 at MMIO 0x81422000 (irq = 5, base_baud = 115200) is a 16550A [ 54.755771] dw-apb-uart.2: ttyS2 at MMIO 0x81420000 (irq = 7, base_baud = 115200) is a 16550A [ 115.902885] ttyS2 - failed to request DMA Why is ttyS0 disabled and again reenabled? How can I enable it/use it properly? How can I get my login shell on serial? What am I missing?