I made a test item, opened a doc pane.  In the doc pane, did Menu > File > Open > Open Web Page.  Pasted a URL, in this case, just as an example, "https://www.voidtools.com/".  The page loaded in the doc pane.  (Having trouble getting some other pages to load, but this one did.)  The URL appears in the field ItemHTMLFile.

Now I'll try to save this page into internal SNDB storage.  So Menu > File > New > New HTML Document.  I select Option 2, ""Copy the content of the external document into a new document".  This operation fails.  

Now I use Open Web Page and paste in the same URL.  This time I try File > New > New HTML File.  I thought I might get the same message that an external document was loaded, do I want to convert it?  (This time not to an internal document, but to a local HTML file instead of a remote URL.)  Nothing seems to happen.  ItemHTMLFile still points to the remote URL, not to a local HTML file.

I reload the URL as above, and this time I select File > New > New MHT File.  This time a conversion dialog message pops up, and I select Option 2, "Convert the Current HTML Document to MHT".  This time it works, and ItemHTMLFile now points to a local MHT file.

I'm glad the MHT option works - do "New HTML Document" (internal storage) and "New HTML File" (external local HTML file) need some fixing?