Error - 'The currency of account n does not match the currency x of this journal when posting a revaluation'

The error is raised because the default Nominal Exchange Difference account set in the currency record does not match that currency. You can check the defaults by going to System, Currencies and opening the currency record. The field in question is Nominal Exchange Diff.
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To resolve the problem, assign an exchange difference account of the same currency as the default - creating one if necessary.