Sudden Input/output error

Been working ok for many months, but suddenly started throwing this error;

/vertical backup --threads 4 --email --no-quiesce
2019-09-03 09:32:31.728035 INFO PROGRAM_VERSION Vertical Backup 1.3.2
2019-09-03 09:32:32.448085 INFO LICENSE_INFO Licensed to Tickett Enterprises Limited; expires on 2020-08-29
2019-09-03 09:32:32.504421 INFO STORAGE_CREATE Storage set to /vmfs/volumes/syn-01/
2019-09-03 09:32:32.547382 INFO SNAPSHOT_GETALLVM Listing all virtual machines
2019-09-03 09:32:32.887207 INFO BACKUP_VM Backing up dc1, id: 1, vmx path: /vmfs/volumes/SSD/dc1/dc1.vmx, guest os: windows9S                                                                                                                erver64Guest
2019-09-03 09:32:33.036824 INFO BACKUP_PREV Last backup at revision 351 found
2019-09-03 09:32:33.316804 INFO SNAPSHOT_POWER Virtual machine dc1 is powered on
2019-09-03 09:32:33.316979 INFO SNAPSHOT_REMOVE Removing all snapshots of dc1
2019-09-03 09:32:35.195501 INFO SNAPSHOT_CREATE Creating a new virtual machine snapshot for dc1
2019-09-03 09:32:38.368844 INFO BACKUP_UPLOAD Uploaded file /vmfs/volumes/SSD/dc1/dc1.vmdk
2019-09-03 09:32:38.416087 INFO BACKUP_UPLOAD Uploaded file /vmfs/volumes/4c2e0dc0-a2afa1cc-16eb-0025903c61d8/dc1/dc1_1-00000                                                                                                                3.vmdk
2019-09-03 09:32:38.642349 INFO BACKUP_UPLOAD Uploaded file /vmfs/volumes/4c2e0dc0-a2afa1cc-16eb-0025903c61d8/dc1/dc1_1-00000                                                                                                                2.vmdk
2019-09-03 09:32:38.793680 INFO BACKUP_UPLOAD Uploaded file /vmfs/volumes/4c2e0dc0-a2afa1cc-16eb-0025903c61d8/dc1/dc1_1-00000                                                                                                                1.vmdk
2019-09-03 09:32:38.901299 INFO BACKUP_UPLOAD Uploaded file /vmfs/volumes/4c2e0dc0-a2afa1cc-16eb-0025903c61d8/dc1/dc1_1.vmdk
2019-09-03 09:32:39.030601 INFO BACKUP_UPLOAD Uploading file dc1_1-000001-delta.vmdk
2019-09-03 09:32:39.030980 INFO RESTORE_THREAD Using 4 uploading threads
2019-09-03 09:33:01.353595 INFO BACKUP_UPLOAD Uploaded file dc1_1-000001-delta.vmdk 46.68MB/s 00:00:22
2019-09-03 09:33:01.353773 INFO BACKUP_UPLOAD Uploading file dc1_1-flat.vmdk
2019-09-03 09:33:01.482480 INFO RESTORE_THREAD Using 4 uploading threads
2019-09-03 11:04:37.249267 ERROR FILE_READ Failed to read file '/vmfs/volumes/4c2e0dc0-a2afa1cc-16eb-0025903c61d8/dc1/dc1_1-f                                                                                                                lat.vmdk': Input/output error
2019-09-03 11:04:37.249428 INFO SNAPSHOT_REMOVE Removing all snapshots of dc1
2019-09-03 11:04:50.482182 INFO EMAIL_SEND A notification email has been sent to blah@blah.com

We do have another job which excludes the biggest disk which is still working OK.

I saw the suggestion in the other thread; -no-chunk-listing but that didn’t help.

Any ideas?