OK. So it still looks like you are trying to connect over HTTPS if you are being warned about risk. Do you have http access turned off and only allow HTTPS? I can’t find a setting that actually turns off HTTPS. I hope Tahoe is not forcing https for all connections. Actually, doing some quick searches, it looks like that is EXACTLY what is happening.
OS X Tahoe is forces http to https redirections. It looks like your NAS must have outdated certificates or something so it fails. As I am not running Tahoe, I can’t guide you on how to disable this feature, but that’s what you should do. It’s dumb of Apple to force secure connections everywhere. They aren’t needed on a LAN.