A K2 Category linked to a menu creates the following error:
404 - COM_CONTENT_ERROR_ARTICLE_NOT_FOUND

A single K2 item will be displayed when linked to a menu. If a Category link is selected to be shown on the Item View, the same error appear when clicking the Category link.

A Category can though be listed using the K2 Content module.

I have read on the net that SEF can cause this but I have tried that.

Regards
Gorm