Hi and welcome to SMF.
Do you have any boards that don't belong to a category? Like if they are on the forum "root" directly?
Hi, thanks
Yes, I suppose I do. I had no idea that was an issue.
I did some googling and noticed the error could be related to mysql strict mode, so I disabled strict mode and the error went away. It is currently importing topics.
Before I mark the issue as solved, should I still be concerned about the boards with no category parent?