"PQO Evaluation Failed" with no description on DataMart

[Anonymous]

I was changing the Power Query code on a DataMart, and this error displays when trying to save:

The DataMart has a few sources, but my changes touched neither the sources nor any tables using them. I added a small table (11 records with 3 columns), and added a column to an existing table. Both of those tables are generated purely by Power Query M code and don't reference any external data directly or indirectly.

What could be the problem? How can I even get more information about my problem?

[Anonymous]

I solved it by checking "Allow combining data from multiple sources."

Having one of the tables reference another, even though neither referenced external sources, caused the issue.

I don't know why the error message had no description though. The poorly designed error messages make Power Bi a nightmare to use at times. 

[LokeshT]

The issue persists, even after enabling "Allow combining data from multiple sources.".