Device registration fails on core20 without an informative error message

I’m experimenting with core20, but my device fails to register in the store

pi@ubuntu:/etc/systemd/system$ sudo snap change 24
Status  Spawn               Ready               Summary
Undone  today at 08:56 UTC  today at 08:56 UTC  Run prepare-device hook
Done    today at 08:56 UTC  today at 08:56 UTC  Generate device key
Error   today at 08:56 UTC  today at 08:56 UTC  Request device serial

......................................................................
Request device serial

2021-09-23T08:56:20Z ERROR cannot deliver device serial request: unexpected status 400

I see that prepare-device hook added all the settings I need:

device-service.headers.Api-Key  <redacted>
device-service.url      https://serial-vault-partners.canonical.com/v1/
registration.body             serial: <redacted>
registration.proposed-serial  <redacted>

I’ve read snapd code in github and I see that it handles error details from the store, if there are any.
But in this case it looks like that the store is not returning any details.