This is madness, sysupgrade requires an absolute path and cannot cope with a relative path:
BusyBox v1.34.1 (2025-10-28 06:59:39 UTC) built-in shell (ash)
____ _ ___ ____
| _ \ _ _| |_ / _ \/ ___|
| |_) | | | | __| | | \___ \
| _ <| |_| | |_| |_| |___) |
|_| \_\\__,_|\__|\___/|____/
-------------------------------------
Teltonika RUT2M series 2025
-------------------------------------
Device: RUT241
Kernel: 5.15.186
Firmware: RUT2M_R_00.07.18.3
Build: ec1ef51550e
Build date: 2025-10-28 07:44:31
-------------------------------------
root@RUT241:~# cd /tmp/
root@RUT241:/tmp# sysupgrade -b backup.tar.gz
Fri Dec 12 12:25:41 UTC 2025 upgrade: Saving config files...
Failed to create the configuration backup.
root@RUT241:/tmp# sysupgrade -b /tmp/backup.tar.gz
Fri Dec 12 12:25:50 UTC 2025 upgrade: Saving config files...
root@RUT241:/tmp#
The test was done directly after a factory reset.
Here is a RUT240 with latest firmware after a factory reset working just fine:
BusyBox v1.34.1 (2025-07-22 04:55:23 UTC) built-in shell (ash)
____ _ ___ ____
| _ \ _ _| |_ / _ \/ ___|
| |_) | | | | __| | | \___ \
| _ <| |_| | |_| |_| |___) |
|_| \_\\__,_|\__|\___/|____/
-------------------------------------
Teltonika RUT2 series 2025
-------------------------------------
Device: RUT240
Kernel: 5.4.291
Firmware: RUT2_R_00.07.06.19
Build: 869421ab6a6
Build date: 2025-07-22 06:03:22
-------------------------------------
root@RUT240:~# cd /tmp/
root@RUT240:/tmp# sysupgrade -b backup.tar.gz
Updating profile default
Tue Jul 22 05:07:57 UTC 2025 upgrade: Saving config files...
root@RUT240:/tmp#