Decryption failed because the HMAC could not be validated - error on Migration d2d dashboard page
I am not sure this will help you, but I ran into this error while using webforms: 


  Decryption failed because the HMAC could not be validated.


My issue was I tried to encrypt a webform field after there was already unencrypted data saved for that field in the database. So when I tried to view the results of my webform, I would get the above error because it was trying to decrypt data that was not encrypted. 

Perhaps this is what is happening when you try to migrate?

My solution was to manually clear the data from the database. Once I did that, I was able to get to the webform results page in the drupal admin and clear the data from there to be sure all evidence of it was gone. Because I am in development this was an ok solution.

I am guessing clearing the data will not work for you, but perhaps this will help point you in the right direction.