Thank you for your tips… but think about, what I wrote exactly:
I right-clicked on INBOX - this is a folder!.
Then I see context-menue with “export this folder …” ( German: “Diesen Ordner exportieren”)
If right-click on a simple email context-menue “Save as …” the same problem: *.zip with 0 Bytes.
Upgrade to Neth7 is no option (now)
Some weeks ago, I tested upgrade Neth6 >> Neth7, but there were errors - I think I was the cause.
I will try again rsync-method with 2 machines, and If it OK, I will try again "Folder export"
Of course there is a daily backup with ALL (incl. sogo) and I tested restore on Neth6 >> OK
It should be a good thing, if USERS are able to archive their email-boxes - /var/lib/nethserver/vmail/ is no option for users.