Mauno,
in my group settings it is said: "Archives are visible to subscribers
only", ... or what should I change to be get them non-visible?
That indicates that on the Settings page of the group, in the Privacy section, the Visibility option is set to one of the choices that says "private messages".
Which is the correct setting for what you want.
... but seems, that they are visible to non-members, too.
That setting controls only the Messages.
The group's home page is always visible to the public. If your group has enabled the wiki and/or calendar those have their own controls.
Is that a bug
If you mean that the Messages content is visible to non-members that would be a serious bug. Can you give us any more detail about how you are seeing them?
Shal
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