[SalesForce] Foreign key external ID: not found for field in entity Account

When trying to upsert contacts and relate the Contact to Account using an external ID on Account I am getting the following error

Foreign key external ID: ABC12 not found for field DBExt_ID__c in entity Account

I have an Account record in SF (in the correct instance) where the External Field DBExt_ID__c = 'ABC12'

DBExt_ID__c is marked as external id in Object manager:Account

DBExt_ID__c is a text field.

On Contact there is field called DBExt_ID__c which is a lookup to Account.
On Contact the external field is called UserExternal_ID__c which is a lookup to Account.

Edit
Using Dataloader to do the upsert of Contacts, using ContactsExternalID.

CSV

**ParentAccountExternalID   ChildContactExternalID  FirstName   LastName**

    ABC12                    User123                 Tim         Song
    PQR34                    User345                 Jim         Haas

Mapping

ChildContactExternalID =UserExternal_ID__c 
LAST_NAME=LastName
FRST_NAME=FirstName
ParentAccountExternalID=Account.DBExt_ID__c  

(also tried ParentAccountExternalID=DBExt_ID__r. DBExt_ID__c)

The problem is with mapping the Contact to the Accounts ExternalID so that i will be able to relate the Contact to correct Account via ExternalID.