Hello,
I have checked login with the default Storefront theme and see the same issue https://gyazo.com/12dc50ac4b972883fb74ae2fb0c6f800
I suppose you need to contact Dokan Pro plugin author to fix the issue because if the login/registration does not work even with the default WP theme we can do nothing. We are able to help only in case problem appears with our theme only.
Regards