That helps a lot! Thanks!

According to this data, there are several controls pointing to fields with "." within the control source (Data tab in control's Properties), including "Products.ProductId" in the NorderDetails form. Was there a reason for this? If the form OrdersDetails uses the table [Order Details] as its record source, that table has its own field simply named "ProductId". Therefore, the control source should typically just read "ProductId".

If your intention was to grab the ProductId field from the Products table instead, I don't think you can actually use it this way, as the Products table is not part of the record source you're using. In fact, I'm not sure why you'd need the version of ProductId from the Products table if ProductId from the [Order Details] table shares a relationship with that value. If you have some records entered in the [Order Details] table, try opening the form NordersDetails just by itself, with no filters, and browse through the records. If the ProductId control comes up with "#Name?", the control source is configured incorrectly. The same might be true of OrderId in NordersMaken and BlowerId in NordersDetails, so make sure the controls are actually storing data where you want them to.

Regardless of where the data above is pointed, putting a period in the name of a control unnecessarily complicates things in VB Code. If you have a textbox named "Products.ProductId" and try to reference it in code such as "Products.ProductId = 5", Visual Basic is looking for a control named Products with a property named ProductId. But since a control named Products doesn't exist, you get an error. I would rename every control containing a period to something simpler. Oddly enough, the control you've apparantly been trying to access, "Order_Details.ProductId", is actually named "Serial" right now, so the proper code for what we tried earlier should have been:
[VBA]
Forms!NordersMaken.NordersDetails!Serial = Me.ProductId
[/VBA]