HTML Extension 404 page not found

I recently reinstalled Lectora Desktop. When I opened one of my old projects to edit some CSS in the CSS HTML extension, I found no CSS lines there. Instead, a "404 page not found" message was displayed in an empty editing box. The CSS is still working as before, but I can't click and start typing. I soon discovered that the same error appears even if I add a new HTML extension, regardless of whether it is CSS, JavaScript, custom div, or something else. Attached is a screenshot of the error itself. Has anyone experienced a similar issue? The same error appears in a new project as well. I am using Lectora v21.5.3 (12504).