Optimizely CMS 13 and errors when creating pages after upgrade

Optimizely CMS 13, the latest version of Optimizely CMS, was released on March 31, 2026.

After upgrading a website from Optimizely CMS 12 to CMS 13, I encountered two strange errors.

The first error was that I was unable to create new pages and instead received the message: “Unable to create page.”

I also saw the following exception:

System.ArgumentNullException: 'Value cannot be null. (Parameter 'model')'

The second issue was that while images could be uploaded without any problems, I was unable to delete them.

After some trial and error, a discussion on the Optimizely Slack brought my attention to the following lines of code:

app.UseEndpoints(endpoints =>
{
    endpoints.MapRazorPages();
    endpoints.MapContent();
    endpoints.MapControllers();
});

If I remove endpoints.MapRazorPages(), or simply rearrange the lines like this, everything works:

app.UseEndpoints(endpoints =>
{
    endpoints.MapContent();
    endpoints.MapRazorPages();
    endpoints.MapControllers();
});