XPages seems to like everything signed by the same ID, and that ID has to be pretty much killer. My experience (I am not an Admin) has been the signing ID needs all rights to everything. I recommend to my clients that they should generally sign the DB with the serverID. Then have the group LoacalDomainServers have all rights on all servers.
I have had instances where most of a DB was signed by the server, but some were signed by me (even though I'm part of a group Notes_Admin) that has all rights and got an error. If I sign the DB entirely with my ID or entirely by the serverID it works. Can be very frustrating until you get that figured out. I have found that a 403 error is always an issue of signing.