assertedDate (http://hl7.org/fhir/us/core/SearchParameter-us-core-condition-asserted-date.html) is defined as

Condition.extension.where(url = 'http://hl7.org/fhir/StructureDefinition/condition-assertedDate').extension.valueDateTime

I think the expression should be

Condition.extension.where(url = 'http://hl7.org/fhir/StructureDefinition/condition-assertedDate').valueDateTime

(removing the 2nd extension in expression)

because Condition.extension.where(url = 'http://hl7.org/fhir/StructureDefinition/condition-assertedDate') returns matched extension

extension.valueDateTime return the dateTime value for simple extenion.
extension.extension.valueDateTime is the dateTime for a complex extension.

This search parameter is in both v5 and v6