Your queries are different to mine, but the problem may be the same - that Jet doesn't know about your form objects, etc.
You could try evaluating the object references first, like this:
dim strSQL as string
strSQL = "UPDATE [VME Chassis Contents] SET [Slot Number] = " & Me.Combo44 & " WHERE [Serial Number] = " & Me.CmboRepairHistory
CurrentDb.Execute strSQL