Error of type system.security.permissions.uipermission at textbox.focus().

I use the following code to move a textbox along with the cursor at runtime and get user input:

Private Sub pictureBox1_MouseClick(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles pictureBox1.MouseClick
Try
If textflag = True Then
If TextBox4.Text = "" Then
Me.Panel2.Location = New Point(e.Location)
Panel2.Visible = True
TextBox4.Visible = True
Me.ActiveControl = TextBox4
Me.TextBox4.Focus()
End if
End if
Catch ex As Exception
MessageBox.Show(ex.Message)
End Try
End Sub

It works fine, but when I host it in server and access it from client, I get system.security.permissions.uipermission at textbox.focus().

Any suggestions?

Thank you.