By searching elsewhere on the Internet, namely at the ASP.Net forums, I discovered that the problem is due to my having changed all the languages, locales & localization settings to use en-GB rather than en-US, but not having this file:
<DNNPath>\Providers\HtmlEditorProviders\Ftb3HtmlEditorProvider\ftb3\Languages\en-GB.xml
This is easily rectified (when you know how!) by copying the en-US.xml file & calling it en-GB.xml.
I guess as the file contains English language terms for various things (e.g. <resource name="BulletedList">Bulled List</resource>), there should actually be a copy of this file for all English-speaking locales, like Great Britain, Australia etc., even though they're identical to the en-US.xml file. If DNN shipped with these files, this error would not have happened.
What really confuses me, however, is that it worked some of the time! How can that be?
I hate to moan about a community such as this, but I have posted a number of issues into these forums (fora?) that actually make using DNN impossible until they are resolved, & yet most of the time I've received absolutely no responses. Is that normal? Does anyone ever get their posts answered? I fear my colleague was right when he said the trouble with open source software is that you're on your own when it comes to getting support.