I tried
@posts = Post.page(params[:page]).per_page(10)
and
@posts = Post.paginate(:page => 1, :per_page => 10)
Posting several years later, in case someone else faces the same challenge.
kaminari-mongoid was released in 2016, and is currently maintained.
https://github.com/kaminari/kaminari-mongoid
All the goodness of Kaminiari for Mongoid, including appropriate handling of Mongoid::Criteria results, which was possibly the cause of the OP's error.