Made a configuration error in specifying the force group in smb.conf - put a @ in front of the group command.
force group = @groupname
should have been
force group = groupname
Not picked up by testparm and causes Linux client to have issues mounting it:
Error: Failed to mount Windows share: Invalid argument
Please select another viewer and try again.
Removed the extra @ in the force group command and the problem goes away. Would have expected the testparm util to pick up this issue. Took a long time to diagnose the issue and it would have been nice for testparm to pick this up.
I think it's possible to have groups containing (and starting with"@"), so it wouldn't make sense to make testparm complain about the "@".
Unfortunately it's confusing that some parameters like "valid users" which accept both, users and groups, use the "@" to specify groups and others that accept groups only don't...
Closing out bug report as won't fix for now. Please feel free to re-open if more discussion is needed.