I didn’t read through your CSS, so I can’t promise it’s not an issue there, but when this happens to me, 100% of the time, I’ve found that I either had a missing closing div tag or one too many closing div tags somewhere. Is this happening on every page of your site (check sidebar.php) or just the index and maybe other pages that list posts (check index.php)?
Good luck, stuff like this can be infuriating!
At some point you have made some changes to your layout or something has if this theme is essentially the bp-default the #sidebar is in the wrong position for a floated sidebar following after content elements.
#sidebar needs to be inside #container not outside – move #sidebar just before the closing tag for #container.
Thank you for your answers. I fixed it now, something simple as an incorrect margin.
Hmm yet you did move the sidebar back to where I told you it needed to live! 
now I have the same problem again but “only” in Chrome and Opera and only on the index page, activity and every other page seems to be just fine. Maybe it’s because of that thing hnla mentioned earlier, or has it got something to do with unclosed divs on the index page other than css?
