I copied your codes into my ThisWorkbook_Startup method. Everything works fine in my side. Cell3 is restricted to be One to Five from the comboBox.
And in your side, which line throws the expcetion, get_Range or Validation.Add? Which language version of Office you are using? And, have you tried to install the latest service pack for Office, to see if this fixes the issue?
I am trying to add range validation on button click in VSTO Excel document-level addin. If I am using VBA button control and corresponding macro - no problems occur. But if I try to do it via Win Forms button control and VSTO -
Exception from HRESULT: 0x800A03EC appear.
I am using ADO.NET to get appropriate values for validation list.