Uh this looks indeed like an issue with Flarum.
When javascript is disabled the list of discussions is shown 20 at a time with pagination links. However it seems Flarum continues to add "next page" links even when there are no more posts ?
Google must be following these next links again and again. Of course it will never reach the end ?
EDIT: I just checked, this has already been fixed for beta8. You'll need to wait for this release to have this fixed