There's a minor omission in the ManagerMenuDefault.xml in the initial upload of this release.
The setting adminsettings.groups is missing a new section named 'Checkout' for the new cartthumbs settings.
To fix, edit the adminsettings.groups setting in the back office and replace the contents with this:
<root>
<notifications text="Notifications"/>
<payment text="Payment">
<pricing text="Pricing"/>
<checkout text="Checkout"/>
<shipping text="Shipping"/>
<gateways text="Gateways"/>
</payment>
<catalogue text="Catalogue">
<images text="Images"/>
<products text="Products"/>
<categories text="Categories"/>
</catalogue>
<security text="Security"/>
<system text="System">
<backoffice text="Back Office"/>
<data text="Data Management"/>
<reports text="Reports"/>
<misc text="Misc"/>
<tabs text="Tabs"/>
</system>
</root>