I think you already have a fix for this, but for the benefit of others, here's some more information.
The problem is related to issues with the Windows operating system's Cryptography subsystem. If this is not working correctly, then both Monarch and Data Pump (and indeed, a host of other applications) cannot encrypt and decrypt files.
However, this is not always the solution, in many cases there is corruption and it must be refreshed. However, it is worth trying first.
With Data Pump, you also need to ensure the same procedure is performed for the service account under which the Data Pump service runs.
There is a folder called 'Crypto'. Rename this to 'CryptoOLD' and then the operating system will repair it when it is next used.
Has anybody experienced a similar issue?