I found where is the problem
I found out that the FB, when a file field is included in the form for uploading like an attachment, makes this field to be required, even if you don't check it as required. In this case, when this attachment is included in the UE, the form is not sent without reporting an error and returns in the page where the form is. To resolve the problem, I deleted the validation in the server behavior pannel as also I deleted the appropriate line in the WA validation. If the file field (attachement) is checked as required from the beginning then this problem doesn't appear.