I have a WordPress multisite installation with these websites:
A portal: http://www.gprsoftware.nl which links to
- http://www.gpradvies.nl
- http://www.gprstedenbouw.nl
- http://www.gprspecials.nl
- http://www.gprgebouw.nl
- http://www.gpronderhoud.nl
The problem occurs on all pages of gprgebouw.nl and gpradvies.nl but not on the other domains. Here is a screenshot of the homepage of gprgebouw.nl

What happens:
- the right sidebar slides under the content area
- the bar-graph icon image is increased in size
- grey borders of the sidebar are extended to the top of the content area
Especially point 3 makes me believe that somehow the content area is not closed properly, but I cannot find this in the HTML source.
The weirdest part is that this only happens on the mentioned websites and not on the others although they share all of the Javascript and plugins and most of the CSS (only some colours are different).
Of course all is fine in browsers like Firefox.
Any ideas what is causing this?
I’m using selectivzr and modernizr for IE lte 8.
On http://www.gprgebouw.nl you have a missing closing div tag here: