Import error: Error opening mailbox
Now this error ususally occurs when the user does not have Mailbox Import Export permissions. This can be assigned to a user using the following PowerShell command:
New-ManagementRoleAssignment -Role "Mailbox Import Export" -User "PSTImportUser"
However in my case this was not the problem. I found out the hard way that you must have a 64bit version of Outlook installed on the PST Importer server. My version of Outlook was 32bit hence causing the error.