Getting "Error writing to output file" when running "apt-get update"

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Nextcloud version (eg, 20.0.5): 24 — I think
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04.4 LTS
Apache or nginx version (eg, Apache 2.4.25): not sure
PHP version (eg, 7.4): not sure

My Nextcloud instance died during an auto-update a couple of weeks ago. I’ve been trying to find the cause, and I think it may have something to do with whatever has broken apt-get update. I get the following errors:
Err:5 https://packaging.gitlab.io/janus/focal focal InRelease
Error writing to output file - write (28: No space left on device) [IP: 35.185.44.232 443]

df -h
Filesystem Size Used Avail Use% Mounted on
udev 2.9G 0 2.9G 0% /dev
tmpfs 593M 2.0M 591M 1% /run
/dev/mapper/ubuntu–vg-ubuntu–lv 39G 38G 0 100% /
tmpfs 2.9G 0 2.9G 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 2.9G 0 2.9G 0% /sys/fs/cgroup
/dev/sda2 976M 207M 702M 23% /boot
ncdata 1.7T 1.1T 539G 68% /mnt/ncdata
/dev/loop1 384K 384K 0 100% /snap/bpytop/254
/dev/loop0 384K 384K 0 100% /snap/bpytop/255
/dev/loop5 56M 56M 0 100% /snap/core18/2409
/dev/loop3 44M 44M 0 100% /snap/certbot/2192
/dev/loop2 44M 44M 0 100% /snap/certbot/2133
/dev/loop4 56M 56M 0 100% /snap/core18/2344
/dev/loop6 62M 62M 0 100% /snap/core20/1494
/dev/loop7 62M 62M 0 100% /snap/core20/1518
/dev/loop8 68M 68M 0 100% /snap/lxd/22526
/dev/loop9 68M 68M 0 100% /snap/lxd/22753
/dev/loop10 47M 47M 0 100% /snap/snapd/16010
/dev/loop11 47M 47M 0 100% /snap/snapd/16292
tmpfs 593M 0 593M 0% /run/user/1000

I’ve tried the various suggestions for removing apt cruft, all to no avail. At this point, I’m tempted to do a clean Nextcloud install to fix this, but I don’t know that I feel comfortable migrating my configuration to another install.