So, I just added a new Windows client (2.4.11) to my urbackup server (running 2.4.13 on Linux), and mistakenly the excludes were set up wrong, so after the latest incremental, it had backed up around 460 GB of data I didn’t want it to.
So I adjusted the excludes, ran another backup, and once that completed, manually triggered a delete of the old backup Now™ through the web UI.
Once that finished, the statistics hadn’t updated, so I clicked “recalculate statistics”, and waited until the “recalculating” activity had gone away.
After running to completion, it now reported “-424972000000 bytes” for used by files on that one client (all others are normal) (and 0 for images on said client, since I have those turned off).
Re-running the statistics update again produced normal results.
(The total space also appears to have been off by approximately the negative amount involved - perhaps a check for negative space before summing might make sense?)