Yup, I used dmesg and journalctl extensively, but there was nothing pointing to an issue there. I made a mistake of rebooting the pi before looking at them more carefully though.
When this was happening, all the relevant services were running (active). I am just curious why nginx would not start because “address was already in use” - I did not have any other running services which I read could interfere (apache, lighttpd).
Anyway, the first command only shows 1 thing right now:
bcm2708_fb soc:fb: Unable to determine number of FBs. Disabling driver.
Same as here. Weird reboot behaviour on RPi 4 B - #2 by Joulinar
Doesn’t seem relevant as it’s a headless server.