Some weeks ago I installed a new machine, using BTRFS for all partitions:

$ cat /etc/fstab
# /etc/fstab
# Created by anaconda on Sat Nov 26 20:35:38 2022
#
# Accessible filesystems, by reference, are maintained under '/dev/disk/'.
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info.
#
# After editing this file, run 'systemctl daemon-reload' to update systemd
# units generated from this file.
#
UUID=9e7d27dc-4997-4786-a59b-f499b7c2da05 /                       btrfs   subvol=root,compress=zstd:1 0 0
UUID=3c00938e-c696-4625-943c-6fe5de440291 /boot                   ext4    defaults        1 2
UUID=EA20-5E75          /boot/efi               vfat    umask=0077,shortname=winnt 0 2
UUID=9e7d27dc-4997-4786-a59b-f499b7c2da05 /home                   btrfs   subvol=home,compress=zstd:1 0 0
UUID=9e7d27dc-4997-4786-a59b-f499b7c2da05 /var                    btrfs   subvol=var,compress=zstd:1 0 0
UUID=7172fdcf-9fbf-40d5-9c70-320161db653d none                    swap    defaults        0 0
UUID=b249dccf-03f9-4ffd-839d-70723d158f1d none                    swap    defaults        0 0
UUID=f8b27592-96c1-4d78-a48a-ce0cf418d15f none                    swap    defaults        0 0

$ df -h
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs        4.0M     0  4.0M   0% /dev
tmpfs            16G     0   16G   0% /dev/shm
tmpfs           6.3G  1.9M  6.3G   1% /run
/dev/nvme0n1p3  403G   11G  391G   3% /
tmpfs            16G  140K   16G   1% /tmp
/dev/nvme0n1p3  403G   11G  391G   3% /home
/dev/nvme0n1p3  403G   11G  391G   3% /var
/dev/nvme0n1p2  868M  409M  398M  51% /boot
/dev/nvme0n1p1  100M   18M   83M  18% /boot/efi
tmpfs           3.2G   92K  3.2G   1% /run/user/1003

This morning, BOINC complained that there was 0.00 MB available to BOINC (which, as you can see above, shouldn't be a problem at all, from the looks of it). No tasks were being downloaded and there were only 14 tasks left, while I had set both MCM1 and OPN1 to 64.

In the Disk tab of BOINC Manager, it said:
"Free, available to BOINC: 0 GB",
"Free, not available to BOINC: 389 GB".

So, in BOINC Manager, I unchecked all 3 'Disk and memory' checks:

[ ] Use no more than ...
[ ] Leave at least ...
[ ] Use no more than ...

(They were all set to their default values.)

Then, after a few seconds, the space constraints were lifted by BOINC and tasks started flowing in again.

The situation, half an hour later:

used by BOINC: 12.70 GB
free, available to BOINC: 389.57 GB
free, not available to BOINC: 197.02 MB
used by other programs: -201744.00 KB

(Alas, I don't have much time now anymore, I have to leave for the day, but I wanted to get this out at least. Don't know when I'll be back at this particular computer.)