Solution for consistent “authentication failed” messages, courtesy of Bignstrauss (Can't join server - "authentication failed"):
- Open Internet Options on your computer
- Go to the Advanced tab
- Scroll down to Security
- Check the box for “Use TLS 1.2” (enable it!)
I did this on my server and was immediately able to connect. It was already enabled on my client PC, so I don’t know if it needs to be enabled on both server and client or just server.