Of course, it's because the form has already closed and the code is referring to the combobox.
Change it to:
'Close logon form and open splash screen
Dim iEmp as Long
iEmp = Me.cboEmployee
DoCmd.Close acForm, "frmStartUp", acSaveNo
If Nz(DLookup("Admin", "Technicians", "[ID]=" & iEmp),0) Then...