Errors can be logged without also logging asserts by enabling -scriptlog. More asserts will be logged when enabling -debugscripts. Game data loading logs (gui, modifiers, traits. Since patch 2.4.1 only logged if -debugscripts is enabled. In game notifications and output of log commands. debug -debugscripts -scriptlog -fullhistoricalsetuplog File More logging can be activated by using command line arguments (via Steam game properties / define launch options):
These are overwritten every time the game starts.
The game stores various log files in your CK2 user folder ( ~\Documents\Paradox Interactive\Crusader Kings II\logs\).