This one is a head scratcher. I'm testing upgrading from 8.2 to 11. On the 9.0 to 10.0 leg of the journey, I see this on every admin screen in version 10:
[Error loading the layout]
Could not load type 'CMS.PortalControls.CMSAbstractLayout'.
I'm stumped.
CMS.PortalControls.CMSAbstractLayout
moved to CMS.PortalEngine.Web.UI.CMSAbstractLayout
in Kentico 10. The API changes were quite wide reaching. Did you make sure that you team the code upgrade tool?. The release notes etc can be found on devnet. https://docs.kentico.com/k10/installation/upgrading-to-kentico-10