Hi,
I had the exact same problem lately with 1.2.12
The problem occurred after validating a dialogue box. It was like Kanboard is freezing, so i interrupted that action and the Database error occurred.
Some user have interrupted a saving command. It is an important bug in my opinion !
That should not occur at all !
I had to restore the last export of the database. So, i lost some recent data. It is not acceptable.
All i can recommend to you for now, is to make exports of the database every day at least until the problem is solved.