As far as I am aware EPiServer CMS 6 does not need session for editing (we run sites with ASP.net session state switched off). If a user's log in has timed out then they would be sent to a log in page on pressing submit so it might not be anything to do with session time out issues.

1. How are you users logging in and what role/membership providers are you using?
2. Are you doing anything with System.Web.Cache or Application level items that could be being removed as a result of an application restart/recycle? 
3. Do you have any custom code that does anything with page save events? 

Questions, questions :)!

1) We are using a custom membershipprovider for single sign-on, so that would explain why a login page is not displayed even after the session and/or the auth cookie has expired.

2) Nope

3) Yes, we are. This was the first place we looked, but no exceptions or anything out of the ordinary.

Hello Anders

Without a detailed look at the code its difficult to say what could be causing the issue. But my gut feel is the custom membership provider. Assuming you are using forms authentication then it could be to do with the fact the rolling time outs on the auth ticket don't work exactly in the way you might assume they do. This post describes things in detail here:

http://weblogs.asp.net/andreaszenker/archive/2009/07/10/slidingexpiration-notreally-timeout-notwhatithinkitis.aspx

I wonder if the auth ticket has timed out which means EPiServer thinks the user is anonymous therefore wont allow any data to come back from the API when viewing in edit mode. Hence blank content pages after save and view?

Let me know how things go.

David