May 26 23:00:44 backupformat/tgz: Attempt 3 failed. Will retry: tarPack pipeline error: ENOENT: no such file or directory, open '/mnt/cloudronbackup/snapshot/app_10da5f93-af48-4972-ae9d-4f416962d3ee.tar.gz' 
May 26 23:01:04 tasks: updating task 11012 with: {"percent":9.695652173913043,"message":"Uploading backup snapshot/app_10da5f93-af48-4972-ae9d-4f416962d3ee.tar.gz (peer.rotovegas.nz)"}
May 26 23:01:04 backupformat/tgz: addToPack: error adding ./dump.rdb file ENOENT: no such file or directory, open '/mnt/cloudronbackup/snapshot/app_10da5f93-af48-4972-ae9d-4f416962d3ee.tar.gz'
May 26 23:01:04 backupformat/tgz: BoxError: ENOENT: no such file or directory, open '/mnt/cloudronbackup/snapshot/app_10da5f93-af48-4972-ae9d-4f416962d3ee.tar.gz'
    at file:///home/yellowtent/box/src/backupformat/tgz.js:67:24
    at Sink._continuePack (/home/yellowtent/box/node_modules/tar-stream/pack.js:42:5)
    at Sink._destroy (/home/yellowtent/box/node_modules/tar-stream/pack.js:121:10)
    at WritableState.updateNonPrimary (/home/yellowtent/box/node_modules/streamx/index.js:213:16)
    at WritableState.update (/home/yellowtent/box/node_modules/streamx/index.js:195:72)
    at WritableState.updateWriteNT (/home/yellowtent/box/node_modules/streamx/index.js:564:10)
    at node:internal/process/task_queues:149:7
    at AsyncResource.runInAsyncScope (node:async_hooks:214:14)
    at AsyncResource.runMicrotask (node:internal/process/task_queues:146:8)
    at process.processTicksAndRejections (node:internal/process/task_queues:103:5) {
  reason: 'FileSystem Error',
  details: {}
}
May 26 23:01:04 backupformat/tgz: tarPack: packed 0 files
May 26 23:01:04 backupformat/tgz: Attempt 4 failed. Will retry: tarPack pipeline error: ENOENT: no such file or directory, open '/mnt/cloudronbackup/snapshot/app_10da5f93-af48-4972-ae9d-4f416962d3ee.tar.gz' 
May 26 23:01:04 backupformat/tgz: [Error: ENOENT: no such file or directory, open '/mnt/cloudronbackup/snapshot/app_10da5f93-af48-4972-ae9d-4f416962d3ee.tar.gz'] {
  errno: -2,
  code: 'ENOENT',
  syscall: 'open',
  path: '/mnt/cloudronbackup/snapshot/app_10da5f93-af48-4972-ae9d-4f416962d3ee.tar.gz'
}
May 26 23:01:24 tasks: updating task 11012 with: {"percent":9.695652173913043,"message":"Uploading backup snapshot/app_10da5f93-af48-4972-ae9d-4f416962d3ee.tar.gz (peer.rotovegas.nz)"}
May 26 23:01:24 backupformat/tgz: BoxError: Failed to add ./dump.rdb: already finalized or destroyed
    at file:///home/yellowtent/box/src/backupformat/tgz.js:73:39
    at new Promise (<anonymous>)
    at addEntryToPack (file:///home/yellowtent/box/src/backupformat/tgz.js:63:12)
    at addPathToPack (file:///home/yellowtent/box/src/backupformat/tgz.js:110:23) {
  reason: 'FileSystem Error',
  details: {}

to me it reads like its blaming the truenas destination, but the truenas reports no problems. and the file app_10da5.... is 9Gb and created on the 24th May. Should I just delete the files in the snapshot folder and let it re-create them ?

Oh, dolphin also complains about the file not existing, when I try to open or delte it, so I gues its just corrupt. /case closed.