Influxdb backup not working as expected

Hi all,

I am using Influxdb v1.7.8 running in a docker-compose environment. Everything works fine, I have successfully inserted several million points using the HTTP api and visualized them in Grafana.

But while trying to implement a backup routine I need some help. I execute the backup as follows:
docker-compose exec -T influxdb influxd backup -portable /var/lib/influxdb/backup

The command is executed successfully, 18 files get created. But all of them are nearly empty (1KB), the data is definately not contained in these files.

What is happening here?

Thanks and best regards
Keith

Additional information:
The tar.gz files are all broken as it seems:

root@a89c12d810a1:/var/lib/influxdb/backup# tar xzf 20191008T152834Z.s11.tar.gz
tar: Unexpected EOF in archive
tar: Unexpected EOF in archive
tar: Error is not recoverable: exiting now

Regression of https://github.com/influxdata/influxdb/issues/9613?

Another finding: All gunzipped tar files are either 512 bytes or 1024 bytes in size. The rest seems to be lost.