apt update: Could not handshake: An unexpected TLS packet was received

Check if the proxy is also set for https. Since curl is working with https, I assume, that somewhere a https_proxy variable is set (e.g. ~/.bashrc). apt requires a proxy configuration in /etc/apt/apt.conf or /etc/apt/apt.conf.d/. You need to specify the proxy for all protocols:

# e.g. in file /etc/apt/apt.conf.d/05proxy
Acquire::http::proxy "http://192.168.0.1:3128/";
Acquire::https::proxy "http://192.168.0.1:3128/";
Acquire::ftp::proxy "http://192.168.0.1:3128/";

Also keep in mind, if the proxy configuration is only set for the sdnc-dev user, you need sudo -E to expose the environment to the root user.