Where do I find boot log in Debian 8 (to debug autostart issues)?

sudo journalctl in Jessie outputs the boot window details including the Warnings and Errors. I have found it to be most helpful in fixing errant packages that systemd starts or attempts to start. It is located at /bin/journalctl.

Debian Jessie is using systemd, so there is no longer an /var/log/boot file.

You can still obtain a lot of information on the boot process with the command dmesg.

But if you use fbautostart, as I suppose, your problem should appear during the start of the window manager. As fbautostart is relatively trivial the best way, is probably to start by commenting everything, then commenting out, i.e. uncomment, each lines of your configuration files, step by step until you find your problem.