I bought a theme but the support period has expired and they no longer respond to my emails.
The authentication popup did not work on the proceed to checkout button (mini-cart & cart).
I found this piece of code in app/design/frontend/[Vendor]/[theme]/Magento_Theme/layout/default.xml :
<referenceBlock name="authentication-popup" remove="true"/>
After override the layout, the popup appears correctly except on the home page. I try to remove all modules, also try with a fresh install.