Hi Fabby,
Looking at your plugin list I would try to uninstall the unofficial plugin
since it seems the only one in the list that is acting on new users (TL0) and has not been updated for over 2 years
Uninstall it, rebuild the site and let us know how it goes.
Have you noticed if the closing is taking place in a single category or in several categories?
Have you noticed if it happens only in private or public categories or both?