What is the target storage?
Click on the text that says that if failed, on the Schedules tab, in the Status column; the log will open in the new browser tab. See what’s there. If inconclusive – add -d
global flag to prune, run it again, and check a new, more verbose log.
This is expected false positive, since the prune did not complete – the snapshots that were to be pruned are now incomplete. You can get rid of that false positive somewhat by adding -fossils
flag to check.