Error creating backup disk image 512
I had the exact same issue using a combination of ZFS Samba, could create files on the mounted drive when testing but trying to create sparsebundles failed.
Turns out as others have pointed out that it might have to do with filesystems used, I had to add vfs objects = catia fruit streams_xattr zfsacl for it to work.
source: https://ogris.de/howtos/freebsd-backup-server.html