Could not load file or assembly 'Microsoft.SharePoint.Portal, Version 15' error after migration (4255747)

Could not load file or assembly 'Microsoft.SharePoint.Portal, Version 15' error after migration

If migrating between different Office 365 subscriptions a site is migrated however in the target environment when the site is opened this error is seen in the UI:

Could not load file or assembly 'Microsoft.SharePoint.Portal Version 15. The system cannot find the file specified.

Microsoft is currently in the process of upgrading many O365 subscriptions to SP2013 architecture. This error occurs because the source site is most likely to be on a content database that has been upgraded to SP2013 however the target is SP2010.

Within the homepage a reference still exists that is pointing to the version 15 of the assembly instead of the correct version 14. This error occurs because the target server cannot locate the correct assembly to render the page.

When the page has been migrated to the target environment it must be amended in SharePoint Designer.

1. Open the target site in SharePoint Designer
2. Navigate to the home.aspx page and view the code.
3. Search for the string 'Microsoft.SharePoint.Portal' and you should find one in line 1 or 2 of the page code.
4. Change the Version 15 to Version 14.
5. Below this entry there may also be an entry for 'Microsoft.SharePoint.Chart Version 15' Change this reference to 14 as well before saving the file back to SharePoint.
6. The page will now correctly open within a browser.