A circular reference (also called loop) happens when one table is reachable by two or more ways. As in QlikView tables are linked when two or more fields are named alike, the likeliest reason is that there are more than two tables with the same field names that link to each other.
Customers: Orders: Calendar:
CustomerID --> CustomerID
Date OrderDate --> OrderDate AS Date
Customers links to Orders that links to Calendar that in turn links to Customers.