Fedora silverblue 38: flatpack update fails due to min-free-space exceeded

In which filesystem is the space needed?

[pippo@fc38sb ~]$ df -h
Filesystem Size Used Avail Use% Mounted on
devtmpfs 4.0M 0 4.0M 0% /dev
tmpfs 1.9G 0 1.9G 0% /dev/shm
tmpfs 773M 9.9M 764M 2% /run
/dev/nvme0n1p3 19G 17G 1.5G 92% /sysroot
tmpfs 1.9G 16K 1.9G 1% /tmp
/dev/nvme0n1p2 974M 208M 699M 23% /boot
/dev/nvme0n1p3 19G 17G 1.5G 92% /var
/dev/nvme0n1p3 19G 17G 1.5G 92% /var/home
vmhgfs-fuse 466G 223G 243G 48% /var/mnt/hgfs
tmpfs 387M 108K 387M 1% /run/user/1000
/dev/sr0 3.0G 3.0G 0 100% /run/media/ltosolini/Fedora-SB-ostree-x86_64-38
[pippo@fc38sb ~]$ flatpak update
Looking for updates…

    ID                                   Branch          Op          Remote          Download
  1. [✗] org.fedoraproject.Platform f38 u fedora 910.3 MB / 910.3 MB

Error: Can’t pull ref runtime/org.fedoraproject.Platform/x86_64/f38 from child OCI registry index: ostree-tar: Failed to handle file: ostree-tar: Failed to import file: Writing content object: min-free-space-size 500MB would be exceeded, at least 29.3 MB requested
Updates complete.
error: There were one or more errors

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[pippo@fc38sb ~]$ df -h
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs        4.0M     0  4.0M   0% /dev
tmpfs           1.9G     0  1.9G   0% /dev/shm
tmpfs           773M  9.9M  764M   2% /run
/dev/nvme0n1p3   19G   17G  1.5G  92% /sysroot
tmpfs           1.9G   16K  1.9G   1% /tmp
/dev/nvme0n1p2  974M  208M  699M  23% /boot
/dev/nvme0n1p3   19G   17G  1.5G  92% /var
/dev/nvme0n1p3   19G   17G  1.5G  92% /var/home
vmhgfs-fuse     466G  223G  243G  48% /var/mnt/hgfs
tmpfs           387M  108K  387M   1% /run/user/1000
/dev/sr0        3.0G  3.0G     0 100% /run/media/ltosolini/Fedora-SB-ostree-x86_64-38

What you show displays /sysroot, /var, and /home all on /dev/nvme0n1p3 with 92% of 19G used. This seems to indicate that you only selected to create a 20G partition to contain the OS, which is normally inadequate for more than testing purposes.