To use Sitecore items to handle errors, allowing different error items for different managed websites, or to use an external URL to handle errors, the RequestErrors.UseServerSideRedirect setting cannot be true.
Shared knowledge on Sitecore
To use Sitecore items to handle errors, allowing different error items for different managed websites, or to use an external URL to handle errors, the RequestErrors.UseServerSideRedirect setting cannot be true.