Ok I solved the issue, thanks to this reference: https://www.userelements.com/err_too_many_redirects/
The problem was that I was thinking to use the login page of WooCommerce so I did not set a login page for UM and this was creating the redirect issue.
I did a conflict test, keeping active only the Ultimate Member plugin and Easy WP SMTP (otherwise I do not receive the email with the activation link)… But still the same issue 🙁
