I'm using Silverstripe 3.0.3 and have this same issue on 2 separate sites I've tested. Content managers can only add new pages if the pagetype is Page. Any custom page that extends page leads to a "Forbidden" error. Here are a few screenshots to illustrate what I mean.
The groups permissions - http://cl.ly/image/0U0o2t412C3I
The simplest of pages - http://cl.ly/image/0n0M2K3E1A0I
The resulting error - http://cl.ly/image/2a3v2i0h0h0L
I've tried adding the canCreate() but no dice. Anyone have this same issue? Any ideas?
I think I've narrowed down the issue. Looks like it's tied to the "Who can create pages in the root of the site?" option. Once that is removed content managers can add any pagetype… When they do it's added to the root of the site despite properly selecting the "Under another page" option properly. Is this a known bug?