Since a recent patch, the inclusion of _pageID in queries causes Gantt charts to fail if they contain a |group by= parameter that's not in a 1:1 relation with _pageID. This is due to backlinks behavior that automatically adds a _pageID field to the Gantt query.

To reproduce, create Gantt data with several entries stored on the same page, and give them some sort of category tag; then group by that category. The query will fail.

(Yaron has access to a specific example on a private wiki.)