Posts are “not found 404”

Hi all,

I have an issue with my posts. they are not show up in my website, not even in the “preview” in wordpress! I tried to create a new post and when i tried to preview, before publish was okay, but when i published it, preview was again “not found”!

I contacted my host and we tried several things, but without working. Also we tried deactivate all plugins but the same thing happened. They told me that is issue of my theme. Does anyone has any idea how to help me with that??

Thanks in advance.
Yiannis

The page I need help with: [log in to see the link]