post and categories link not working

Hi,

I have recently changed the template of my blog and have switched off the “Use the visual editor when writing” from the Users – Profile menu. And now I have this problem – on the homepage everything is fine, but when I try to go to a specific category or post it gives me and HTTP 404 – Not found message (the URL is the exact one as it should be)

Please advise what to do in order to make it work again?

Here is my blog for reference:
http://www.intellectasrl.com/wordpress

Thank you in advance!

Christina