A recent support incident showed that some of our documentation didn’t make it clear just what you can and can’t do with master pages. There are two different types of sites in Project – PWA (the Project Web Access sites) and PWS (the project workspace sites). For the PWA sites you are limited to making use of the default.master we already supply to give any extra pages the same look and feel, but you are not able to edit these default.master pages. We also block SharePoint Designer from opening these sites – so you cannot use this tool against a PWA site. You can however use SharePoint Designer against the PWA workspace sites – and edit the default.master.
The reason for this difference is that it would be easy to break our application by editing our pages – and this would not make life good for a support engineer (and indeed for the customers who manage to break things!). Also remember that even though the workspaces can be modified you shouldn’t add extra instances lists of the specific project type lists of issues, risks, dependencies and documents. Details from these default lists feed through to the reporting database – if there are duplicates then this will break the workspace reporting feature.
This topic is covered at http://msdn.microsoft.com/en-us/library/ms504195.aspx and this highlights another great way you can add to the sum of human knowledge on MSDN. BruceVB added some community content based on his experience – and our own Jim Corbin has added a further comment. Although I wouldn’t necessarily agree that our original document is incorrect – it is certainly not complete in telling you what you can and cannot do. But even being able to use our default.master on new pages does make it easier to build custom Project Web Access solutions – even if you cannot edit the default.master.
If you do try and update the PWA default.master through Site Actions, Site Settings then you may see the error message "The enablesessionstate attribute on the page directive is not allowed in this page."
You would need to reset to the site definition to get the site working again.
Please do use the community content features on MSDN – it certainly does help us to understand how we can improve our content – and thanks again BruceVB for bringing this particular problem to our attention.