I share the error message. Maybe that explains the origin of the problem :
ERROR : unexpected status
INFO: starting new backup job: vzdump --storage backup --compress gzip --mailto
xxxx@xxxxx.fr --quiet 1 --all 1 --mailnotification failure --mode snapshot
INFO: Starting Backup of VM 102 (qemu)
INFO: status = running
INFO: update VM 102: -lock backup
INFO: VM Name: sagecoala
INFO: include disk 'virtio0' 'local-zfs:vm-102-disk-1' 100G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating archive '/rpool/backup/dump/vzdump-qemu-102-2017_11_04-08_00_02.vma.gz'
INFO: started backup task '784e16ee-8eb7-418a-bd11-f7eeb0edf98c'
INFO: status: 0% (107479040/107374182400), sparse 0% (15081472), duration 3, 35/30 MB/s
INFO: status: 1% (1096679424/107374182400), sparse 0% (73871360), duration 46, 23/21 MB/s
INFO: status: 2% (2151415808/107374182400), sparse 0% (114479104), duration 95, 21/20 MB/s
INFO: status: 3% (3224174592/107374182400), sparse 0% (150573056), duration 151, 19/18 MB/s
INFO: status: 4% (4295229440/107374182400), sparse 0% (169799680), duration 199, 22/21 MB/s
INFO: status: 5% (5386141696/107374182400), sparse 0% (200286208), duration 259, 18/17 MB/s
INFO: status: 6% (6442844160/107374182400), sparse 0% (205770752), duration 307, 22/21 MB/s
INFO: status: 7% (7522353152/107374182400), sparse 0% (219734016), duration 356, 22/21 MB/s
INFO: status: 8% (8601862144/107374182400), sparse 0% (248082432), duration 414, 18/18 MB/s
INFO: status: 9% (9677570048/107374182400), sparse 0% (280268800), duration 455, 26/25 MB/s
INFO: status: 10% (10749476864/107374182400), sparse 0% (370061312), duration 503, 22/20 MB/s
INFO: status: 11% (11817582592/107374182400), sparse 0% (540852224), duration 547, 24/20 MB/s
INFO: status: 12% (12891127808/107374182400), sparse 0% (691322880), duration 594, 22/19 MB/s
INFO: status: 13% (13987414016/107374182400), sparse 0% (975564800), duration 639, 24/18 MB/s
INFO: status: 14% (15044706304/107374182400), sparse 1% (1232211968), duration 679, 26/20 MB/s
INFO: status: 15% (16135618560/107374182400), sparse 1% (1314230272), duration 718, 27/25 MB/s
INFO: status: 16% (17218928640/107374182400), sparse 1% (1336397824), duration 779, 17/17 MB/s
INFO: status: 17% (18282708992/107374182400), sparse 1% (1380962304), duration 829, 21/20 MB/s
INFO: status: 18% (19351011328/107374182400), sparse 1% (1393278976), duration 920, 11/11 MB/s
INFO: status: 19% (20404240384/107374182400), sparse 1% (1451364352), duration 968, 21/20 MB/s
INFO: status: 20% (21480210432/107374182400), sparse 1% (1478610944), duration 1019, 21/20 MB/s
INFO: status: 21% (22567059456/107374182400), sparse 1% (1485434880), duration 1082, 17/17 MB/s
INFO: status: 22% (23646568448/107374182400), sparse 1% (1509847040), duration 1145, 17/16 MB/s
INFO: status: 23% (24726077440/107374182400), sparse 1% (1513549824), duration 1195, 21/21 MB/s
That's all.