Optimizely CMS 12: “The page may have been changed by another user (…)” error

What might cause this error

Recently, when I was working with Optimizely CMS 12 (formerly EPIServer CMS), I encountered a confusing error message that seemed unrelated to what I was doing:

Could not save property. The page may have been changed by another user. Please reload the page.

I gradually reverted my recent changes to experimentally find out that the culprit was a property added to the content type I developed. Here’s a simplified version of the code:

public enum Color { Red, Green, Blue }

[SiteContentType(GUID = "11111111-2222-3333-4444-555555555555")]
public class MyNewPage : PageData
{
    [SelectOne(SelectionFactoryType = typeof(EnumSelectionFactory<Color>))]
    public virtual Color LogoColor { get; set; }

    [SelectOne(SelectionFactoryType = typeof(EnumSelectionFactory<Color>))]
    public virtual Color? LogoColorNullable { get; set; }
}
Code language: C# (cs)

What is wrong with it? The CMS 12 platform cannot handle the Nullable variant of the enumerable types. Hence, the “?” character here causes the issue, and the error is being handled by displaying a generic message, which is unrelated to the root cause of the problem.

Other causes of this error

It looks like the above error message is displayed whenever a POST operation fails to save data and receives the 409 Conflict response from the CMS backend server, regardless of whether it was caused by another user saving the content before you did.

For future cases like that, I’m leaving us a hint that it’s worth digging intothe browser’s console logs to find a hint on the root cause there: