An object list only shows "[...]Subquery returns more than 1 row". Problem

Problem

When calling an object list, I do not get the list of objects, instead I get an error message that starts with

Subquery returns more than 1 row

instead.

Solution

The message suggests that there are single-value categories with more than one entry in the system. It is sufficient to use the Delete Duplicate Single-Value Entries button under Administration → System Tools → Cache / Database. All affected object lists can then be called up again.