[SalesForce] Cannot Query Synchronized Data Extension

I have created a Query to consolidate data from 2 Synchronized Data Extensions into one Sendable DE, but upon running it I get an "Automation failed due to system error".

At first I thought my query was written incorrectly but neither Validate nor Hive SQL Syntax Checker returned an error, so I created a super-simple Query to test, and to my surprise I got the same error.

The super-simple Query I created was just:

SELECT Email 
FROM Contact_Salesforce

Seeing as how undescriptive the error message was, I created a ticket with SF to which they replied:

…I looked on the backend and noticed an error that says "must exist
and be active". Is the Synchronized Data Extension in the same
Business Unit? If not you will need to make sure ent. is before the
Synchronized Data Extension.

I would move your case over to their team but it appears you have our
Standard Support Entitlement. As you have Standard support, we will
close the case at this time.

We only have 1 (Parent) BU with no Child BUs. So using "ent." before the name of the DE is not useful. Nevertheless I tried it and got the message

"The 'Ent.' prefix can only be used to query data in a parent account
from a child account"

So I´m not sure what´s going on, or why I get this error message AFTER running the Query, when everything, from the Query itself to the DE, is configured correctly.