Converter can only return null

Hi everybody!

I have a really strange problem and I need some help from the experts=)

I have an SelectOneListBox with values which come from a bean. This works pretty good. Now I wanted to get the selected value as property of my bean. Because it is a complex type I added a converter to the project. This also worked pretty well. Because of a change in the business logic I had to change the the object which gets passed to the listbox and to the converter. I there the problems started.
At first I had the behavior that non of the frontend values was set in my bean but I got no error message at all. I discovered that I can make all properties except the for the selecteditem work if my converter returns only null. As soon as I return the correct type (the object has the correct type, I checked this) nothing works but I get no error. I expect that the converter is the cause of my problem. But I have no approach what I can do to make it work. That is the main reason I am posting here.
Are there any restrictions for the objects returned by a converter that I am not aware of (e.g. is a getter and setter for every field mandatory)? Or has anybody an idea what the cause of this behavior might be?

Thank you for your help!
M