OK, I had the same problem and had already typed in a long reply to ask how to solve this.
As you can see I deleted that long reply.
When testing, as I did, I used the "Forum URL Management"and set them to category1 and forum1,DO NOT
use the terms category or forum when testing, that is when this error comes up.
Use aaabbb and cccddd or whatever, but NOT category and forum.
Just lost 6 hours over this, so hopefully any other new installers of SEO can avoid losing time over this.