Observed using a Nitrokey Pro2 and gnupg2 (2.2.4-1ubuntu1.2).
Also seen using gnupg 2.2.10 and brainpoolp512r1 [1].
# gpg2 --export-ssh-key $key_id
gpg: export as ssh key failed: Unknown elliptic curve
# gpg2 --list-keys $key_id
pub brainpoolP256r1 2019-03-01 [SC] [expires: 2019-05-30]
$key_id
uid [ unknown] username <user@email.net>
sub brainpoolP256r1 2019-03-01 [A] [expires: 2019-05-30]
sub brainpoolP256r1 2019-03-01 [E] [expires: 2019-05-30]
sub ed25519 2019-03-01 [S] [expires: 2019-05-30]
[1] Issue also observed using gnupg 2.2.10 and brainpoolp512r1.
Retrieved from the Nitrokey support list:
https://support.nitrokey.com/t/gpg-with-nitrokey-storage-2-using-brainpoolp512r1-unable-to-export-public-ssh-key/1395