It's a template revert issue, revert the forumhome template and I think you'll be sorted.
Or, if that template is heavily modified, do what I did and look for the bits that cover the opening and closing of the sidebar, and replace those with the equivalent bits from the "default" version of the template.
This happens because of a bug fix that was implemented in 4.1.10 to stop people with the sidebar collapsed getting a quick "flash" of it if they refreshed the page, it was visually ugly like that. Works great now.