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?
Thanks in advance,
Andrew