To reproduce:
- Add default .net RichTextBox to a blank form.
- Add a contexmenu strip (again .net one) to the RichTextBox.
- Spell check using RadSpellChecker and you will notice that the menu cannot be opened after that.

Workaround:
void richTextBox1_MouseDown(object sender, MouseEventArgs e)
{
    if (e.Button == System.Windows.Forms.MouseButtons.Right)
    {
        richTextBox1.ContextMenuStrip.Show(this.richTextBox1, e.Location);
    }
}