I created a model, tree anf form views and an action. When I try to access the new module I receive the above message (sort of, is in romanian, not sure exact the english version but must be similar). If I assign a single view, it's work :

field name="view_mode">form
 

But ehen I try 

field name="view_mode">tree,form

does not work :(

What can be the explanation?

Thank you very much!