The error message says the direct error is that your SMB server is refusing the connection. I’m not running 21 yet (nor do I have any SMB externals, yet), but I’m in tech support mode here, bear with me.
You said you did a fresh install, I take it that means you re-added the settings to that install manually, not as a copy-paste from the old to the new?
Out of curioisity, what’s your SMB server? Samba? Something Windows? Part of me wonderrs if the SMB library in 21 is using a different protocol version that it’s not happy with.