I installed Transmission a long time ago on my Raspberry Pi and couldn't figure out why each time I rebooted my Raspberry Pi I was forced to restart transmission-daemon otherwise Transmission web interface wasn't reachable.

Because transmission-daemon isn't logging to a file by default, I had to search how to enable this.

You have to edit the service launched by systemd located at:

/lib/systemd/system/transmission-daemon.service

And append those arguments to the command that launches transmission-daemon:

--log-debug --logfile /var/log/transmission.log

I replaced --log-info with --log-debug but I'm not sure if this is mandatory. Don't forget to create the file and chown it to debian-transmission user otherwise the daemon will not be able to write it.

I was able to reboot and see transmission logs with this error:

getaddrinfo: address family for nodename not supported (trevent.c:216)

After some research, I found that this error was raised because my Raspberry Pi didn't have an IPv6 address.

As I didn't use IPv6, I disabled it by editing /etc/sysctl.conf and adding this line (credit to Leow Kah Man):

net.ipv6.conf.all.disable_ipv6 = 1

Now, transmission-daemon starts as expected at boot time.