"Are you missing a directive or assembly reference?" Me or not?

Cleaned up and rebuilt everything many times, restarting VS and the computer, but no progress. While fixing some bugs in the open source project QueryCommander, I ran into issue with Debug build but Release doesn't work. I only get this error in Release mode :

CS0246  The type or namespace name 'AxSHDocVw' could not be found (are you missing a using directive or an assembly reference?)
QueryCommander.Help C:\Workspace\QueryCommander_4_0_0_0\QueryCommander.Help\WinGui\FrmHelpBrowser.cs

      

        
        
        
      

    

The link does exist:

And it contains the missing namespace:

The block FrmHelpBrowser.cs

doesn't really have a directive using

, but I'm not sure why this works in mode Debug

and what should be used here:

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using QueryCommander.Help.WinGui.Base;

namespace QueryCommander.Help.WinGui
{
    /// <summary>
    /// Summary description for FrmHelpBrowser.
    /// </summary>
    public class FrmHelpBrowser : FrmBaseContent
    {
        public AxSHDocVw.AxWebBrowser axWebBrowser1; // Error on this line
//----------------^ here

      

        
        
        
      

    

What's missing in release mode?