The navigation pane is where all the objects are represented (forms, reports, macros, modules, tables). A navigation form is a special kind of form.
So I don't know what you mean by a navigation pane form. I guess either you created another form, you see it in the pane, and you added your form to it. Or you've added your form to a navigation form. Assuming the latter, your issue is probably due to not correctly referencing the form which has now become a subform.
Try
Forms![main navigation form].[NavigationSubform].Form
If you didn't change the navigation subform control name, it is probably really named NavigationSubform.
[main navigation form] is the name of your navigation form IIRC (I don't use navigation forms)
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.