No, this isn't the problem (unclosed tags).
I created this post a few days ago and it displayed just fine when I created it.
I haven't made any new posts, nor have I edited any posts in the meantime and now it doesn't display properly anymore.
I haven't changed anything on my site, therefore I conclude that something changed on wordpress.com. I didn't change any theme settings, but suddently my blog picked up some "scroll infinitely" setting that I never had before.
Since the theming error is putting my left sidebar and right sidebar at the bottom of the page, I can never get to them when "scroll infinitely" is turned on, because they keep scrolling off the screen every time I try to get to them. This prevents me from even being able to login through the sidebar.
I navigated directly to the wp-admin URL and turned this off.
This is how I know that WordPress.com did something to my blog that wasn't there before and it's why I suspect that they did something that now prevents the Garland theme from working properly.
Once infinite scrolling is turned off, both the left and right sidebar content appear on the left at the bottom of the page after the main content.