Did you use root’s fcrontab or fcronuser’s fcrontab?
you can try /usr/local/bin/backupctrl iso > /tmp/backup.log 2>&1 to get a log.
just tried it with root’s fcrontab, which gave me the log:
/bin/bash: line 1: bin: command not found
trying sudo /usr/local/bin/backupctrl iso with fcronuser’s fcrontab works well.
Due to the fact that root’s fcrontab on certain updates is rewritten by the system, its recommended to use a separate user for scheduling custom scripts.
A perfect guide for this is here https://www.ipfire.org/docs/pkgs/fcron