amcheck reports holding disk space is being used when it isn't; dumps report "out of holding space in degraded mode"

[imneedham]

I've inherited this Amanda installation from someone else. It's based on the Amanda packages included with the Ubuntu repos. The OS is currently 24.04.3 LTS, but it was probably first installed on Ubuntu 16 or 18. Almost all of the Amanda files are under /etc/amanda.

Amcheck tells me that most of my holding space is full, even though it is actually empty. I can't figure out why it thinks the space is being used.

Amanda Tape Server Host Check
-----------------------------
NOTE: Holding disk '/dumps/amanda/daily': 1722 GB disk space available, using 1622 GB
slot 2: volume 'rdx4-2' is still active and cannot be overwritten
slot 3: volume 'rdx4-3'
Will write to volume 'rdx4-3' in slot 3.
NOTE: skipping tape-writable test
Server check took 5.171 seconds
(brought to you by Amanda 3.5.1)

This apparently results in warnings in my nightly emails:

FAILURE DUMP SUMMARY:
  web-svcs.work.lab /etc lev 0  FAILED [out of holding space in degraded mode]
  win-2.work.lab win_2_systemstate_2 lev 1  FAILED [out of holding space in degraded mode]

I have two configs, daily and monthly. They once shared holding disk (now the path is different), and the NOTE in Amcheck is identical for each.

This started in September when I was traveling... The "tape" needed changed, and then the holding space filled. When I returned I was able to flush everything onto the new "tape", but Amanda persists in thinking that my holding disk is full.

I've tried so many different things I can't keep track of them all. I'm sure there's lots of old cruft in /etc/amanda that can be removed, but I'm hesitant to just start deleting anything else without community advice.

How can I figure out why Amanda thinks 1622 GB is being used?

Thanks.