How to debug gnome-shell session crash?

After messing around a bit with window managers (bpswm, on debian testing, specifically), i cant boot my GNOME session on Xorg anymore (wayland work just fine, but some of my apps are not wayland compatible).

All i get is a full screen adwaita themed error message ( https://photos.app.goo.gl/qfgqhFqrYXvpuwgN9 ), and when i google it, most of the things i get back is very old (like 2012), or not very specific.

I guess there’s a log file somewhere with information, and that it would not something big (probably something i messed up in my user session), but i can’t find where to look.

Any ideas ?

Start by looking at the output of journalctl --user. Quite likely one of the core components crashed and that triggered the failure screen. But there can also be other issues e.g. in the systemd units that cause this.

No matter what the cause is, there should be something in the log about why the fail-whale (the unhappy face screen) is started. It might not be obvious though as to what it is, feel free to paste/upload the full log of the failed session for others to look at.