I have this problem with my site with qtranslate and qtranslate slug plugins installed :
Look at this page:
This is the italian version of my blog . If you try to change the cathegory (eventi garfagnana / ricette della garfagnana)you will notice that you'll get always the same page containing the 2 cathegories together.
Instead, in english (my default language) all is working well.
Can you help me to solve this problem ? I think it is caused by qtranslate.