I also deleted the spaces as you told me in FORCE_NIC and I had the same problem again as if there was no entry.
cat /sys/class/net/enp3s0/operstate also says "up". All services (ssh, SMB, ...) work.
Here is the log output:
Feb 13 21:50:10 NAS autoshutdown[29519]: : INFO: 'autoshutdown-restart(): Autoshutdown starting'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: 'main(): XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: 'main(): Openmediavault-autoshutdown version: 5.1.10'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: 'main(): Script md5sum: 797d77704d47042f9c89017df30894f2'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: 'main(): /etc/autoshutdown.conf loaded'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: 'main(): /etc/autoshutdown.default loaded'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: '_check_config(): ------------------------------------------------------'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: '_check_config(): Checking config: /etc/autoshutdown.conf'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: WARNING: '_check_config(): CHECKCLOCKACTIVE set false not validating UPHOURS and OVERRIDEUPHOURS'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: WARNING: '_check_config(): CHECKPROCNAMES set false not validating LOADPROCNAMES, LPREPEAT, TEMPPROCNAMES and TPREPEAT'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: WARNING: '_check_config(): LOADAVERAGECHECK set false not validating LOADAVERAGE'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: '_check_config(): Kernel 5.9.0-0.bpo.5-amd64 supports power management modes:'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: '_check_config(): Kernel supports: HIBERNATE (suspend to DISK)'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: '_check_config(): Kernel supports: HYBRID (suspend to RAM and DISK)'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: '_check_config(): Kernel supports: SUSPEND (suspend to RAM)'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: '_check_config(): SHUTDOWNCOMMAND is set to: systemctl suspend'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: '_check_networkconfig(): ------------------------------------------------------'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: INFO: '_check_networkconfig(): Find available network interfaces and their IPv4 addresses'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: DEBUG: '_check_networkconfig(): Network interfaces: enp3s0'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: DEBUG: '_check_networkconfig(): Network interfaces status: down'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: DEBUG: '_check_networkconfig(): Network interfaces IPv4 address: 192.168.178.27'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: DEBUG: '_check_networkconfig(): Network interfaces address valid: true'
Feb 13 21:50:10 NAS autoshutdown[29682]: root: ERR: '_check_networkconfig(): No valid network interface found, exiting ...'