Is this the case that RadEditor was used prior to the upgrade? If so, I think that the best solution is to revert back to the original site, switch the HTML Editor Provider to CKE, and then repeat the upgrade. Somewhere in there you also want to uninstall the RadEditor provider. You do that on the Extensions page.

Before the upgrade the site was version 9.0.1 I believe, so it was probably the case that RadEditor was being used. I wasn't in charge of the upgrade to this instance, but other instances were upgraded in a similar way and aren't having the same problem.

Unfortunately, reverting back is not an optio so any fixes will have to be done on the site as it is.

Would uninstalling the HTML editor module and then reinstalling the lastest version work as a fix? I had considered it but since 90% of the site is simply pages with the HTML editor I didn't want to risk something breaking in a worse way.

If you have the Persona Bar available, you can go to Site Settings, Site Behavior, More an using the dropdown there to select the CKE as the default editor.

If you do not have the PB, you can edit web.config to make CKE the default. It should be installed or available as part of the upgrade. If not, separately create a clean install of DNN 9.6.1 and take a look at web.config to see how it should be edited. You'll be section in web.config that specifies the default HTML editor.

Big thanks for your quick reply. I do have access to the PB, so I've checked in Site Settings and the install is already using CKE. I had updated the web config previously as well.

Looking at my Extension settings the HTML module is at version 9.0.2, which is a far cry from the current version, so maybe that is where the problem lies.

Yes, you should have version 9.6.1 of the HTML module installed. You should be able to find the module's installation file in the upgrade package for DNN 9.6.1. Just use the PB Extensions page to install it as a normal module.

Upgrading the module fixed the above problem, but now there are further issues. I'm only able to view Edit Content in the Basic Text Box view. The console is now throwing a "Uncaught ReferenceError: CKEDITOR is not defined" among a few other 500 internal server errors.

Going into Site Settings -> Site Behavior -> More to the HTMl Editor Manager page, it is set to DNNConnect.CKE but there is a message that reads "The current HTML Editor Provider is not supported by this version of the HTML Editor Manager."

If it's useful, the web.config includes:

    htmlEditor defaultProvider="DNNConnect.CKE"
      providers
        clear 
        add name="DotNetNuke.RadEditorProvider" type="DotNetNuke.Providers.RadEditorProvider.EditorProvider, DotNetNuke.RadEditorProvider" providerPath="~/DesktopModules/Admin/RadEditorProvider" settingsControlPath="~/DesktopModules/Admin/RadEditorProvider/ProviderConfig.ascx" 
        add name="DNNConnect.CKE" type="DNNConnect.CKEditorProvider.CKHtmlEditorProvider, DNNConnect.CKEditorProvider" providerPath="~/Providers/HtmlEditorProviders/DNNConnect.CKE/" settingsControlPath="~/Providers/HtmlEditorProviders/DNNConnect.CKE/Module/EditorConfigManager.ascx" 
      providers
    htmlEditor

Using a clean install of 9.6.1, compare versions of all installed extensions, and do the same trick with them.

Thanks Joe.

Comparing this site to another site upgraded at the same time, from the same version and with the same process (in fact, the problem site was a copy of this other site way back in the day): the "parent" site has a working HTML editor using CKEditor and all of the extensions the two sites share are the same version.

I will also compare with a clean install, but the mystifying thing is the DNN sites having these problems are children of this "parent" site which has no problems. They all share almost the same set of extensions are are used for the same function, just with different coats of paint.

It looks like things are breaking in the module as it runs through editorCongifeditortxtContent and hits this line:
blockedKeystrokes: [CKEDITOR.CTRL + 66, CKEDITOR.CTRL + 73, CKEDITOR.CTRL + 85],