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You can restore a backup created on a system with a different architecture. However, you will have to delete the graph databases which are architecture-dependent and need to be recreated.
Do so by executing the following commands after restoring the backup:
/etc/init.d/collectd stop /etc/init.d/vnstat stop rm -rf /var/log/rrd/* rm -f /var/log/vnstat/* /etc/init.d/collectd start /etc/init.d/vnstat start