File sharing authentication fails with SMB works while working with AFP

[DominikHoffmann]

I have a Mac mini running macOS Catalina 10.15.7, which is set to share files. When I turn on both SMB and AFP file sharing, I can authenticate my file sharing client using this URL

afp://user@server.local/​

but cannot, using

smb://user@server.local/​

Why would that be?