..that was all correct. I think that the problem is:

The ddl 'product' is an ASP.NET server control, but it is populated with items on client side instead of on server side. I mean with a javascript function, so, I suppose for that reason, when the page is postback and tries to find the operation that this ddl have to do, (OnSelectedIndexChanged="Certificates"), the ddl server control doesn' t find the 'OnSelectedIndexChanged' property because it hasn' t items on it on server side. It can be possible? If so, what is the solution?