New Post: Add item to list in new item form

Hi.

I´m very new in Sharepoint and i have a trouble that i cant be able to solve. I´ve modified the new item form of a list to be able to add items to the lookup fields thet it have. It works fine and the item is added correctly in the lookup list but when i submit the form to modify the item an error is displayed with no info. Only appears a screen with a "Error ocurred" message. If i reload the form after the item is added to the lookup list, the form is submited correctly.

Any body can help me with this issue?.

Thanks in advance.