Very strange… I can get a RSA public key certificate with the must-staple feature, but when I use an ECDSA key (with must-staple), it malfunctions…?
ECDSA itself is not the problem, but the combination ECDSA and Must-Staple is…
Unfortunately, an internal server error is hard to debug… Could it be this line?
https://github.com/letsencrypt/boulder/blob/master/ra/registration-authority.go#L590