I attempted to join a server that is hosting 7 Days to Die version Alpha 12.5 and kept getting stuck at "Waiting for Server." I was also unable to start a single-player game and received a NullReferenceException error. I disabled my antivirus, Windows Defender, and anything else that could potentially prevent me from logging in to no avail. After the server administrator reviewed the server logs, we determined that the issue was due to my character's hairstyle not existing in Alpha 12.5. Once I selected a default character skin, I had no issues logging into the server.

TL;DR: If you are playing on a legacy version (pre-Alpha 16) server OR single-player, create a new character profile before attempting to join/start a new game.