Here is my code:

Private Sub cmbxPeriod_Change()
Sheets("CEU Database").Unprotect Password:="dataworf"
Sheets("Code").Range("P2").Value = usfReset.cmbxPeriod.Value
Application.ScreenUpdating = False
Sheets("CEU Database").Select
Cells.Select
Selection.AutoFilter Field:=7, Criteria1:=Sheets("Code").Range("P2").Value
Application.ScreenUpdating = True
Sheets("CEU Database").Protect Password:="dataworf",
DrawingObjects:=False, Contents:=True, Scenarios:=True
End Sub

I have used a variation of this code many times in the past without problem.
However, the usfReset.cmbxPeriod.Value is a date. One issue might be that
the cells are already filtered by one criteria.