"Wiki" tab gives missing page error

The "Wiki" tab link works well from the front page, but if you select a different tab and then try to click to the "Wiki" tab it gives you a missing page error.

For example, if you go to the front page of the ubuntu-gr.org site and select the "Κοινότητα" tab and go to this page and then try to go to the Wiki front page by clicking to the "Wiki" tab, it gives you a missing page error. The "Wiki" tab link is supposed to get you to http://wiki.ubuntu-gr.org/ but it tries to direct you to http://www.ubuntu-gr.org/Wiki