I have a two-sidebars layout, but the forum section (sitewide bbforum) the theme doesn't show the left sidebar. You can see it here: http://www.ucurdunne.it/forums/
if set both sidebars in theme settings - both shown on forums page
If this person was using the free version of CC, when you update to the merged versions new option shows up in Theme Settings, General: "Posts lists on archive pages", and the default is "Right Sidebar". Maybe if they change Archives page template here it will solve the issue?
@slaFFik the option for setting a sidebar layout for the archive pages should have an option to select called "default", which should do nothing and it should be set as default also.
And also - seems like there is the content width bug hidden when having different settings for global and archives?
On the site it's not even a Custom Community theme, but another. We can't test it.
i think that issue is old already