"ErrorInvalidUserInfo - A UserId was not valid" error when syncing folder and / or calendar folder permissions (4274155)

When trying to synchronize folder permissions, or Calendar folder permissions, from Source to Target, or Target to Source, the following error stack appears in the Migration Agent for Exchange (MAGE) log:

In this instance, the Calendar or Folder belongs to userA, and they have granted folder permissions to TestUser. 

• For an on-premises Exchange environment, enable the Target account in Active Directory Users and Computers. This is the account that the permissions are granted to; TestUser.
• For Office 365, deselect Sign-In blocked on the account that the permissions are granted to; TestUser.
  • In Office 365 Admin Center, select Users.
  • Select the account that the permissions are granted to.
  • Choose Edit next to Sign-in status in the User Pane.
  • Change Sign-in status to 'Sign-in Allowed'.