Magento 2 - LiveSearch Error: Sorting by category position is only allowed within a single category branch

Question

I have a store with LiveSearch installed, and I get a lot of generic errors in livesearch, such as:

Magento_LiveSearch: An error occurred in search backend.

or

LiveSearchAdapter: An error occurred: An error occurred in search backend.

When I debugg the error, I get this:

Sorting by category position is only allowed within a single category branch.

Could I have a better explanation about this error or how to fix it?

Thanks.