I've noticed this twice, while aiding allies in besieging Banefort. Other castles seem fine. No cheats/edits in that playthrough.
At the start of a fight, a massive amount of script errors is spammed into the event log, causing the log to not display at all. The fight works fine, and after relaunching the game no bugs encountered.
Images:
https://imgur.com/a/LkSEJ