DAVx5 uses an Android WebView to show the Nextcloud login page. According to WebViewClient’s documentation:
Applications are advised not to prompt the user about SSL errors, as the user is unlikely to be able to make an informed security decision and WebView does not provide any UI for showing the details of the error in a meaningful way.
So if you’re using self-signed certificates, please create the account directly in DAVx5 instead of using the Login Flow from the Nextcloud app.
Or get a free Let’s Encrypt certificate, in case that your service is accessible from the Internet…