Today I updated my debian fileserver and surprisingly I couldn't log into a specific share. Only into that one and from windows and linux client.
After some investigations and try and error process it seems that I simply can't have a group called 'stage' and use it in 'valid users' parameters with the '@' or '+' notation.
If so, every log on try (with smbclient) receive a
tree connect failed: NT_STATUS_ACCESS_DENIED
If I rename the group in /etc/passwd to 'gstage' everything works just fine.
Works for me. Feel free to reopen with a log level 10 log if you can reproduce with 4.19.