Hi Lorenz,
the "Permission denied" error is a server error so your backup file is fine and the client was able to parse it properly. The server though won't let your user restore the DB. There should be a log entry in /var/log/qluman/qlumand.log for the attempt giving more information why the server rejected the request.
Restoring the DB is only allowed for the actual admin user as there is currently no right option to allow other users to execute this request. Something we might have to change in the future. Are you logged in as the admin user?
Best regards, Goswin