Monday CRM changed the creator field on the Board object in API version 2027-04. It now returns User instead of User!, so it can be null. Monday CRM says a creator can't always be resolved, and the old non-null type could make a request fail for that board.
With the change, the board is still returned and creator is null. The rest of the board data is not affected.
This is a breaking change for clients that rely on the non-null type, including code generated from the schema. Check for null before reading creator.id or creator.name, and regenerate schema-derived types for 2027-04.