[Solved] Cant select new template after upgrade

[Rattle]

Hi,

I already started upgrading a site from 1.9.3 to 1.11.12 and it appeared to go through cleanly, but I am now unable to change the template on existing pages to a new imported template. As soon as I select it I get a blank CMSMS styled page with the text "404 Error" (but not an HTTP error code). It is however possible to switch the old pages between old templates.

I tried creating a new page and setting the template but again only the old templates are selectable.

Does this sound familiar to anyone - where start in trying to resolve it?

Edit: Nevermind - I noticed the theme I was trying was older than where I started. Newer themes work fine.