After finally getting around to investigating the problem, I easily found a solution in the forums: Apparently, I have to go back to using the rich-text editor, and manually toggle it to use a “source view”.
The underlying problem is likely that Opera (my browser) was previously not supported at all in the editor, which made copy-and-paste of HTML code work per default; however, with some later changes, a partial support was added—and the pasted HTML was now misinterpreted. Toggling the source mode puts the editor back to function as it should.
The following is an automatically generated list of other pages linking to this one. These may or may not contain further content relevant to this topic.