I apologize for not specifying that I had already done that, earlier. To no avail. And our editors are definitely trusted. I’ve done everything correctly, per the few instructions I have been able to find.
With active ‘Enable “unfiltered_html” capability’ option in mind open “Users->User Role Editor”, select “Editor” role and select “Deprecated” group of capabilities and check if “unfiltered_html” capability is really included into the “editor” role.