To reproduce:
 Create a Windows Forms project
- Add a RadDock.
- Add 1 DocumentWindow and 1 ToolWindow to the RadDock control.
- Add a RadTextBoxControl to the DocumentWindow. Give it a height that will allow you to easily scroll.
- Run the application and enter enough text into the text box to make the scrollbar appear.
- Scroll to the bottom of the textbox.
- Minimize the application.

Workaround:
public class MyScroller : TextBoxScroller
{
    public MyScroller(RadScrollBarElement scrollbar)
        : base(scrollbar)
    { }
    public override int Value
    {
        get
        {
            return base.Value;
        }
        set
        {
            if (value < this.MaxValue)
            {
                base.Value = value;
            }
            
        }
    }
}

Change it as follows:
 radTextBoxControl1.TextBoxElement.ViewElement.VScroller = new MyScroller(radTextBoxControl1.TextBoxElement.VScrollBar);