Nextcloud crash - how to find reason?

My NC 16.0.1 installation crashed. I dont know why. How to find out? What log-files shhould I look in?

NC 16.0.1 installation on NAS. Ubuntu 18.0.4, Nginx.

Documentation gives a few starting points:

A little bit less information…

But if it crashes due updating from a minor version a reason could be a third party app which is incompatible with NC 16 and has not been disabled by the updater. In that case you can remove app for app with the occ command and try again.