Looks like org.freedesktop.Platform is currently corrupt and therefore fails to upgrade…
[user@localhost ~]$ flatpak update
Looking for updates…
ID Branch Op Remote Download
1. [✗] org.freedesktop.Platform 20.08 u flathub 111.9 kB / 266.4 MB
Error: While pulling runtime/org.freedesktop.Platform/x86_64/20.08 from remote flathub: opcode close: Corrupted file object; checksum expected='7df828b34c609c1543d730bcf03cd200303ffd91cfe5a67aae60ee78a1626875' actual='cc52edcb5eaa9c8f527cb3167ab9da11cb13714ecf54d3b2b6a1efe31779e85e'
Updates complete.
error: There were one or more errors