I have several pages that are used for administrative purposes and that inherrit from EPiServer.SimplePage to get some basic functionality. Now after upgrading to EPiServer 6 R2 these pages are generating a 404 so I assume I need to set some properties or override some method(s) but I am unsure which. Also by looking at the implementation of SimplePage it isn't obvious to me where the 404 is being generated. Could someone tell me what I need to do to prevent the 404?
Are you using these templates in a page type and creating instances in the page tree? Or are you using them as standard standalone aspx pages?
Standard standalone aspx pages.
Does it need to be? I mean, can it inherit System.Web.UI.Page instead? I'm thinking it's something with the CurrentPage being null that triggers the 404. If you want it to be SimplePage I'm out of ideas. I suggest you enable debug logging to try to find out what happens to it.
I am sure I can re-implement it using System.Web.UI.Page but it would be quite some work I think so I will look some more into if I can find the source of the 404 and update the post if I find something.
My guess is that this is an effect of a bug fix where pages should not get master language fallback automatically. I'd suggest that you try to inherit from EPiServer.UI.SystemPageBase class which resides in the EPiServer.UI assembly. This is the base class that we use ourselves for edit/admin-pages.