Cannot save crontab - No space left on device

[bezik46]

motionEye Version 0.42.1
Motion Version 4.3.1
OS Version motionEyeOS dev20201026

Getting error:

[ Error writing /data/etc/crontabs.1117: No space left on device ]

As per wiki: https://github.com/motioneye-project/motioneyeos/wiki/Tweaks#running-commands-at-regular-intervals-cron-support

both partitions are writable, there is plenty of space left

Filesystem                Size      Used Available Use% Mounted on
/dev/root               213.0M    184.5M     13.5M  93% /
devtmpfs                932.8M         0    932.8M   0% /dev
tmpfs                   997.3M     16.0K    997.3M   0% /dev/shm
tmpfs                   997.3M    248.0K    997.0M   0% /tmp
/dev/mmcblk1p1           29.9M     11.0M     18.9M  37% /boot
/dev/mmcblk1p3            6.1G      6.0G         0 100% /data
/dev/mmcblk1p3            6.1G      6.0G         0 100% /home/ftp/sdcard
/dev/mmcblk1p3            6.1G      6.0G         0 100% /home/ftp/storage
overlay                   6.1G      6.0G         0 100% /usr
overlay                   6.1G      6.0G         0 100% /var/log
overlay                   6.1G      6.0G         0 100% /var/lib
/dev/sda1               457.4G     52.8G    381.3G  12% /data/media/sda1
/dev/sda1               457.4G     52.8G    381.3G  12% /home/ftp/storage/sda1

I need to reboot the unit nightly as it does hang from time to time (and then only power unplug works)
Any ideas why save fails?

I only try to add a single line!

0 2 * * * /sbin/reboot

Thanks