Aged debtor vs control account agrees as a whole but there is a difference between currencies

The most common cause is an 'any currency' customer account where a set of allocated transactions are in different currencies. For example, an invoice, payment, and account adjustment all allocated together, but one transaction is in a different currency:

In this example, the total on the customer account is zero, as is the combined total of the £ and EUR nominal accounts. However, when checking the currency accounts individually, there is a difference — because the £250 adjustment posted to the EUR control account. Both the £ and EUR accounts are out against the debtors for the same currency. To resolve this, either reverse the affected transactions and re-enter them correctly, or use the Corrections module to update the transaction details (if you have access to it).