Why `rear mkbackup` creates rescue image only

Issue

• rear mkbackup doesn't create backup.tar.gz, only a rescue image is created.
• No error messages output, but the 'backup' stage finishes immediately.

2022-04-01 17:13:22.003205784 ======================
2022-04-01 17:13:22.011243704 Running 'backup' stage
2022-04-01 17:13:22.017595400 ======================
2022-04-01 17:13:22.088250031 Including backup/default/010_pre_backup_script.sh
2022-04-01 17:13:22.105603914 Including backup/default/990_post_backup_script.sh
2022-04-01 17:13:22.110943802 Finished running 'backup' stage in 1 seconds
2022-04-01 17:13:22.113177858 Finished running mkbackup workflow
2022-04-01 17:13:22.451947911 Exiting rear mkbackup (PID 5500) and its descendant processes ...