categories link displaying as html on my site

Hi
I have interactive videos inserted in some of my posts. i have posted them in a category named interactive. If I click on the interactive link in categories, in the sidebar, the post is just displaying html. However if you access the same post via films (top nav) or recent posts, then the video player displays properly.

I have contacted the interactive video forum, who suggested that I try the default theme and see if the problem still exists. I did this, but as my theme has three columns and the default theme has two, my categories sidebar link was not visible, so I could not asses the situation.

thanks for any help

Sarah