When setting up the Eloqua connector, an error may be displayed similar to: 

Error occurred during a cryptographic operation.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Security.Cryptography.CryptographicException: Error occurred during a cryptographic operation.

In order to resolve, it could be an issue with your machine keys so check this first. If this doesn't work, please backup your database and run the following command. (This can also work for similar types such as Pardot)

DELETE from tblBigTable WHERE StoreName = 'EPiServer.MarketingAutomationIntegration.Eloqua.Contracts.DataClasses.EloquaSettings'