Yoast SEO – Sometimes forms submit twice
Solution: Change the SEO description for that page
Invisible Defender – Spam protector that breaks Custom redirects (plugin not available anymore on WP.org)
Adminimize – Causes Email Customizer templates to not save changes
W3 Total Cache – With Email Confirmation, users cannot login after confirmation (username and pass not valid error)
Solution: W3 Total Cache Settings -> Object Cache. Move “users” from Global groups to Non-persistent groups.