How to fix the 403 error on preview in Umbraco after upgrading

I ran into an issue today with a site which had recently been upgraded.

The customer reported that the preview didn't work any more.

I did some googling and I found a thread on Our Umbraco which was marked with a solution, but the solution which actually worked for me was further down.

The solution was to delete the folder

The reason for this is because in the upgraded version of Umbraco the files have been moved to the new folder:

So by deleting the old folder it solves the issue.

Thank you to Paul Wright for posting this solution #h5yr