I had the same problem (http://www.episerver.com/en/EPiServer_Knowledge_Center/Developer-Forum2/EPiServer-Developer-Forums-/EPiServer-CMS-version-5/12993/), but it seems that this only happens when you upgrade from RC3 to RC4.
It seems like we can't save and publish pages at all now. Do you have any suggestions of how to resolve this? Right now, it's kind of difficult to keep developing without a solution to this problem.
Thanks,
Christoffer
Apparently the upgrade was missing the db-upgrade-script. Check out the previous topic to learn how you can get this script from the support. (http://www.episerver.com/en/EPiServer_Knowledge_Center/Developer-Forum2/EPiServer-Developer-Forums-/EPiServer-CMS-version-5/12995/)