Hi Alvind,
Thank you for responding! No, I do not have any caching plugins installed.
Meanwhile I decided to recreate the problem in a more controlled fashion. Fortunately the website in question is still in early stages of construction, so I simply created a new WP installation and this time tracked, tested and documented customization step by step. So far there is no problem, I removed the sidebars first thing after activating the theme and they haven’t returned. Will report here with full details if the problem recurs.
