You have a form containing two sub-forms: A top sub-form and a bottom subform.
The current event of the top sub-form sets the recordsource of the bottom sub-form.
On opening the form, when the current event fires in the top sub-form you get Err.Number = 2455.
The reason:  The bottom sub-form hasn’t been loaded yet.
The work around: cut the top sub-form to the clipboard, then paste it right back in.
The effect will be to make the bottom sub-form load first, then the top sub-form and since the current event is in the top sub-form, with the bottom one already in memory, you won’t get the error.