We managed to solve the problem. It was caused by our introduction of an editor for enum properties, as described by Joel Abrahamsson: http://joelabrahamsson.com/enum-properties-with-episerver/
There he suggests this:
ClientEditingClass = "epi.cms.contentediting.editors.SelectionEditor";
Which is clearly no longer supported! The new syntax, which now works, is:
ClientEditingClass = "epi-cms/contentediting/editors/SelectionEditor";
Probably a good idea to update the release notes!
You can find all breaking changes here by version http://world.episerver.com/documentation/upgrading/Episerver-CMS/10/
Thank you for the responses Daniel and Johan!
In hindsight, I see that it was clearly explained under the headline "Remove support for legacy dot notation for dojo modules". I just wasn't able to connect the dots.
Sorry for the noise!
Always good to ask. You wont be alone with that problem :)
Today, we upgraded our EPiServer installation from 9.12.2 to 10.0.2. It seemed to work OK until we tried navigating to the "property view" in the CMS UI for certain pages. Property view never loads, and we get errors in the console:
404 error - failed to load https://localhost/EPiServer/Shell/10.0.2.0/ClientResources/epi.cms.contentediting.editors.SelectionEditor.js
This is followed by internal errors in dojo.js and widgets.js, which seem to be caused by the above error
This seems to happen for all pages (and blocks) which have an image attribute, but not for other pages/blocks.