1. case 1

upsc greenups
Init SSL without certificate database
Error: Driver not connected
systemctl is-enabled nut-server.service nut-driver@greenUPS.service
enabled
enabled
systemctl status nut-server.service
///
Nov 05 12:20:28 kvm010.d51.net upsd[13939]: Can't connect to UPS [greenUPS] (nutdrv_qx-greenUPS): No such file or directory
///
systemctl status nut-driver@greenUPS.service
///
Nov 05 12:22:34 kvm010.d51.net systemd[1]: nut-driver@greenUPS.service: Failed with result 'exit-code'.
Nov 05 12:22:34 kvm010.d51.net systemd[1]: Failed to start nut-driver@greenUPS.service - Network UPS Tools - device driver for NUT device 'greenUPS'.

2. audit

cat /etc/nut/nut.conf
#
MODE=standalone
cat /etc/nut/ups.conf
#
maxretry = "3"
pollinterval = "3"

[greenUPS]
 driver = "nutdrv_qx"
 port = "auto"
 default.battery.voltage.low = "10"
 default.battery.voltage.high = "14"
 override.battery.packs = "1"
cat /etc/nut/upsd.conf
#
LISTEN 0.0.0.0 3493
cat /etc/nut/upsd.users
#
[admin]
 password = Passw0rd
 actions = SET
 instcmds = ALL
 upsmon master

3. reproduce

systemctl stop nut-server.service nut-driver@greenUPS.service
/usr/lib/nut/nutdrv_qx -a greenUPS
Network UPS Tools - Generic Q* USB/Serial driver 0.36 (2.8.1)
USB communication driver (libusb 1.0) 0.46
Using protocol: Q1 0.08
Battery runtime will not be calculated (runtimecal not set)
Network UPS Tools - Generic Q* USB/Serial driver 0.36 (2.8.1)
USB communication driver (libusb 1.0) 0.46
libusb1: Could not open any HID devices: insufficient permissions on everything
No supported devices found. Please check your device availability with 'lsusb'
and make sure you have an up-to-date version of NUT. If this does not help,
try running the driver with at least 'subdriver', 'vendorid' and 'productid'
options specified. Please refer to the man page for details about these options
(man 8 nutdrv_qx).

upsnotify: notify about state 4 with libsystemd: was requested, but not running as a service unit now, will not spam more about it
upsnotify: failed to notify about state 4: no notification tech defined, will not spam more about it

4. solution

disconnect/reconnect the usb port