When trying to export a newsletter through a marketing effort. When you try to export you may be seeing the error message, Error running marketing effort export. Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.
This type of issue occurs when there is a record included in the address that contains more than one primary address. This is not a valid data state for the system, and needs to be addressed by correcting the source record to mark only one of the addresses as primary.