Errors when mounting SharePoint 2010 databases to SharePoint 2013

We're migrating several large SharePoint 2010 content databases into a new SharePoint 2013 farm (in 2010 mode) and when running PowerShell Mount-SPContentDatabase we're seeing hundreds of errors:

Feature upgrade action 'CustomUpgradeAction.EnsureVariationsInfrastructure' threw
an exception upgrading Feature 'Publishing' (Id: 14/'22a9ef51-737b-4ff2-9346-
694633fe4416') in Web 'https://....': Access is denied. (Exception from HRESULT:
x80070005 (E_ACCESSDENIED))

.

Feature upgrade action 'CustomUpgradeAction.VariationsInfrastructure' threw an 
exception upgrading Feature 'PublishingResources' (Id: 14/'aebc918d-b20f-4a11-
a1db-9ed84d79c87e') in Site 'https://....': Access is denied. (Exception from 
HRESULT: 0x80070005 (E_ACCESSDENIED))

.

Feature upgrade action 'CustomUpgradeAction.VariationsInfrastructure' threw an 
exception upgrading Feature 'PublishingResources' (Id: 14/'aebc918d-b20f-4a11-
a1db-9ed84d79c87e') in Site 'https://....': The context has expired and can no 
longer be used. (Exception from HRESULT: 0x80090317)

The only resource we've found that's similar is: http://www.scribd.com/doc/149497400/Migration-de-SharePoint-2010-vers-SharePoint-2013-tuto-de-A-a-Z

However its suggestion that we activate features at the site collection level is not appropriate here because the site collection hasn't been completely mounted to SharePoint at the time the error occurs.

Our project doesn't use SharePoint variations but I suspect that's irrelevant here.

The Google shows no results when searching for "VariationsInfrastructure" or "EnsureVariationsInfrastructure"

Has anyone discovered why this happens and how to resolve it?

In my case I was getting

"Access is denied. (Exception from HRESULT:x80070005 (E_ACCESSDENIED))"

I was able to resolve this issue by making sure that the account under which I am executing the Mount-SPContentDatabase command should have "Full Control" access granted in the web application policy.