Just looking at the error you are seeing, I would assume there is something wrong with the structure of the layout that is causing the error. I would open up your layout in an HTML editor and see if you see any malformed tags that Kentico is construing as potentially malicious.
Kentico has a lot of code inside of it to prevent different types of penetration attacks through malicious code, so I have a feeling something in the layout is being seen as that.