Did this work?
$ python3 /home/pi/.local/lib/python3.4/site-packages/ruuvitag_sensor -f
I wouldn’t be surprised if sometimes RuuviTagSensor won’t kill itself properly when using Crl+C and is still using bluetooth. If you see python3 in the list, kill it.
$ kill -9 [PID]
If that is not the problem, let’s continue with these. Can you see hci0 with this command?
$ hcitool dev
Try to restart all devices
$ sudo service bluetooth restart
$ sudo service dbus restart
$ sudo hciconfig hci0 restart
Maybe some other application is accessing bluetooth, try reboot
As you see we follow basic it-support guideline: have you tried turning it off and on again?