I also had the “400 - bad request” error and I could also resolve it.
In my case, the .tar would contain a folder with the backup.json, addons_local.tar.gz, homeassistant.tar.gz, media.tar.gz and so forth.
That was wrong, I repacked tar file with tar cz ./test.tar ./* with a flat file design (i.e. no folder inside) and it worked just fine.