I've created a new page type "SidemenuPage" and under Templates I've created the SidemenuPage.ss.
My problem is that when I select from CMS, Page type - SidemenuPage and press Save & PUblish, the system automatically selects the "Error Page" page type.
I found that in my sitetree_live DB table, the ClassName field gets the NULL value. If I change this manually to my SidemenuPage class everything is ok...
P.S. I've refresh every time the DB with /dev/build/?flush=1
Not sure if this is the answer you are looking for, but.. If there is something wrong with the page type, it will default to another page type. In other words, the system will not let you create a page type that has an error.