Simple backup access VB.NET

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VB

Visual-Studio

MS-Access

backup

I'm trying to write a code snippet to make a simple backup from the access database in my software.

Everything looks allright (i think) but i get an error when executing the code:

error looks like this: Format of the initialization string does not conform to specification starting at index 0.

I've tried reformatting the string but i can't get it to work.
I was wondering if people here could take a look at the code i wrote?

Thanks in advance for any insights in my error!

What I have tried:

Public Class Backup
    Private backupfolderdestination As String
    Private Sub Main_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        If chkVoorAkkoord.Checked Then
            btnKiesFolder.Enabled = (chkVoorAkkoord.Checked = True)
            btnMaakBackup.Enabled = (chkVoorAkkoord.Checked = True)

        End If
    End Sub

    Private Sub Backup_FormClosing(ByVal sender As Object, ByVal e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
        My.Settings.Save()
    End Sub

    Private Sub btnKiesFolder_Click(sender As Object, e As EventArgs) Handles btnKiesFolder.Click

        If (FolderBrowserDialog1.ShowDialog() = DialogResult.OK) Then
            TextBox1.Text = FolderBrowserDialog1.SelectedPath
        End If
        backupfolderdestination = FolderBrowserDialog1.SelectedPath
    End Sub

    Private Sub btnMaakBackup_Click(sender As Object, e As EventArgs) Handles btnMaakBackup.Click
        Try
            Dim cn As New OleDb.OleDbConnection(My.Settings.GekozenDatabasePad)
            Dim mydatabase As String = My.Application.Info.DirectoryPath

            System.IO.File.Copy(mydatabase & "\PatientenDatabase.accdb", String.Format(backupfolderdestination & "\bac_{0:yyyyMMdd}.backup", Date.Today))
        Catch ex As Exception
            MsgBox(ex.Message.ToString(),
                    MsgBoxStyle.OkOnly Or MsgBoxStyle.Exclamation)
        End Try

    End Sub


    Private Sub chkVoorAkkoord_CheckedChanged(sender As Object, e As EventArgs) Handles chkVoorAkkoord.CheckedChanged

        btnKiesFolder.Enabled = (chkVoorAkkoord.Checked = True)
        btnMaakBackup.Enabled = (chkVoorAkkoord.Checked = True)

    End Sub

    Private Sub IconButton3_Click(sender As Object, e As EventArgs) Handles btnSluitVenster.Click
        Me.Close()
    End Sub
End Class