I have done some tests and can confirm the following points:
The form (BF Registration "Signatures") receives the data, registers the user, and sends the mail to the administrator. The user also receives the email for the activation of his account (Joomla System).
However, the problem with the thank you page persists.
The form consists of 2 pages.
It is loaded in the url
https: // program .......... org.pa/wx3ep49quk9s0s3ewmtayz86wk9sb1gylhp4quns
The thank you page tries to load in this same URL as seen in Image_1
However, the last page of this form is not an attack page nor is it configured as such, see image_2
The thank you page is configured in the submit piece with the following code: