When a new line is placed right after a nbsp, the new line is ignored in the layout. This can be reproduced by importing the following HTML:

First&nbsp;<br/>Second

NullReferenceException when changing themes, the RichTextBox has TrackChanges enabled, and the current user is set.

Workaround: set empty user like this: 

private void RadRichTextBox_ProtectionStateChanged(object sender, EventArgs e)
{
    if (this.radRichTextBox.CurrentUser == null)
    {
        this.radRichTextBox.CurrentUser = new Telerik.Windows.Documents.Model.UserInfo(string.Empty, string.Empty, string.Empty, string.Empty);
    }
}