This was helpful but the result was just the other way around. I have a HFS+ volume on the computer that shares, which enabled me to separately test AFP and SMB sharing. AFP sharing of a folder on the HFS+ volume works (with registered user). SMB sharing of a folder on the APFS volume does not work. It looks like a permissions problem.
Apple's information is theory. I know how to set up sharing, have been using macOS since OS X 10.0 (Rhapsody even). What does happen that for some reason it does not work with SMB. It looks like an authentication issue. Looking on the serving side, I see this in log:
default 10:02:52.426557 +0100 smbd Server requires signing, but not auth-bound to Directory Service
default 10:02:52.427298 +0100 smbd Too many groups requested (2147483647). Can cause performance issues when network directories are involved
default 10:02:52.433288 +0100 smbd Too many groups requested (2147483647). Can cause performance issues when network directories are involved
default 10:02:52.448680 +0100 digest-service digest-request: uid=0
default 10:02:52.448713 +0100 digest-service digest-request: init request
default 10:02:52.452971 +0100 securityd found a non-proper sample, skipping...
default 10:02:52.468923 +0100 opendirectoryd Failed to talk to secd after 4 attempts.
default 10:02:52.472453 +0100 digest-service digest-request: init return domain: ALBUS server: ALBUS indomain was: <NULL>
default 10:02:52.472607 +0100 smbd Server requires signing, but not auth-bound to Directory Service
Seems definitely a problem with smbd configuration.