found it!
basically i made too much effort in the datagridview itself in the form.
Because i made it look like the way i wanted it to, i changed data that was critical to the loading
correct code:
Private Sub Dossiers_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim str As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=F:\GoogleDrive\EINDWERK VBNET\PatientenDatabase.accdb"
Dim con As New OleDbConnection(str)
Dim com As String = "Select tbl_Dossiers.Dos_ID, tbl_Relaties.Rel_Naam, tbl_Relaties.Rel_Voornaam, tbl_Onderzoekstypes.OZ_TypeOnderzoek, tbl_Diagnoses.Diag_Type
FROM((((tbl_Dossiers
Left OUTER JOIN tbl_DossRelatie ON tbl_Dossiers.Dos_ID = tbl_DossRelatie.DR_DossID)
Left OUTER JOIN tbl_Relaties ON tbl_DossRelatie.DR_RelID = tbl_Relaties.Rel_ID)
Left OUTER JOIN tbl_OnderzoeksTypes ON tbl_Dossiers.OZ_ID = tbl_OnderzoeksTypes.OZ_ID)
Left OUTER JOIN tbl_Diagnoses ON tbl_Dossiers.Diag_ID = tbl_Diagnoses.Diag_ID)
ORDER BY tbl_Dossiers.Dos_ID"
Dim adpt As New OleDbDataAdapter(com, con)
Dim dossierset As New DataSet()
adpt.Fill(dossierset, "Dos_ID")
dgvDossiers.DataSource = dossierset.Tables(0)
dgvDossiers.Show()
End Sub
thanks for helping out!