Update.
We deleted the whole database and installed a prior backup, the one from just before we reopened after the big upgrade. We lost that first day's posts after the upgrade.
Last night we got everything set up again, took a snapshot of the disk on the server, backed up the database, and opened the board. The disk usage on the server was a very manageable 36% (out of 80GB), the snapshot was 30GB. The disk storage started rising again as soon as we opened, and it rose to 48% in just five hours. So, we still have a leak.
We closed access the board overnight and the storage flattened out. We reopened this morning and it's rising again.
Marc, updating our access details sounds like a plan. I'll let you know when that's done.