I found this solution at the following website: http://fernando-acero.livejournal.com/
translation by Google
vnstatd[xxxx]: Error: Database load failed even when using backup. vnstatd [xxxx]: Error: Database load failed even when using backup. Aborting. Aborting.
The solution is to open a console as root, go to / var / lib / vnstat and delete all files in it, considering that there are hidden files, for example using mandates, which would eliminate the primary database and the security (hidden file) of the eth0 interface.
rm -rf eth0 rm-rf eth0
rm -rf .eth0 rm-rf. eth0
Then, create a new database using the term.
vnstat -u -i eth0 vnstat-u-i eth0
This command must be repeated with each interface in the system we have, for example, using the usb modem, we must also create a database for PP0 interface through the command:
vnstat -u -i pp0 vnstat-u-i PP0