serial-getty service constantly restarts, seems to read ghost input

Hello,

Today I found that the "serial-getty@ttyS0.service" service restarts every minute. It says "serial-getty@ttyS0.service: Succeeded." in the syslog/journal and then starts again, so it exited with code 0. The restart counter is already at 3085, so it's been like that for 2 days. No output is logged from the service itself. So I looked up its start command and ran it interactively. Again, with no output, after a minute, it returned with code 0.

So I straced that thing and saw that it did lots of things, then print "Password:" somewhere, wait a while and then print "Login timed out after 60 second" and exit with code 0.

strace /sbin/agetty -o '-p -- \u' --keep-baud 115200,38400,9600 ttyS0 linux >agetty.trace 2>&1

Looking further up the trace, it seems that it prints "login:" somewhere several times, and once it reads single characters as input, assembling to the string: "Armbian 20.05.2 Focal ttyS0 \r" It echos every single character, then proceeds to the password prompt. But no further input is read, so it gives up.

Here's the full trace output: https://ygoe.de/s/JSCkKnW7Th90xghl

I'm wondering where that ghost input comes from and who is sending this string, character per character, every time? There's a USB to serial adapter connected to the serial port, but GND is currently left open (or the adapter's LED would light from the TX line voltage) and the USB is not attached anywhere. And that certainly won't generate such a string. When I had last used that USB to serial adapter, I could log in with user name and password just fine.

The device is an Orange Pi Zero with (I think) Armbian_20.05.2_Orangepizero_focal_current_5.4.43. Support log: http://ix.io/2wbG