Have you created a writable clone of the page? Try setting the property directly with page["PageStopPublish"] (or MetaDataProperties.PageStopPublish).

At least previously this built in property had to be set like this I think.

Hi Johan,

The page is a new instance all together. I've tried setting the property directly; still the same exception is thrown direclty. The MetaDataProperties doesn't seem to be accessible. That property doesn't exisist on my PageData object

Thanks for the effort tho

Hmm can you provide some more info about the context in which you are doing this? Is it in the save event? The following code works:

Edit: should be noted, this is with cms version 11

var c = ServiceLocator.Current.GetInstance<IContentRepository>();
var p = c.GetDefault<ArticlePage>(ContentReference.StartPage);
p.Name = "asjdlkajsd";
p.StopPublish = DateTime.Now;
c.Save(p, SaveAction.Publish);

(It is created as expired).

I hook on the 'EventsSavedContent' in a IInitializableModule  
Of course my accutal code is more elaborate

  private void EventsSavedContent(object sender, ContentEventArgs e)
        {
//((PageData)e.Content).StopPublish = DateTime.Now.AddSeconds(-1);
e.Content["PageStopPublish"] = DateTime.Now.AddSeconds(-1);

}

In that event I believe the page has already been created and saved to the repository so based on the code you provide you need to create a new editable version and save that yourself i.e.: 

var newPage = (e.Content as PageData).CreateWritableClone();
newPage.StopPublish = DateTime.Now;
EPiServer.ServiceLocation.ServiceLocator.Current.GetInstance<IContentRepository>().Save(newPage, SaveAction.ForceCurrentVersion | SaveAction.Publish);

I see now, the item is not a writeableClone to begin with

not sure if you have reasons not to, but it sounds like listening to Saving/Publishing event would make things easier to you