Hi @bbox
I've tested it with
www.this-is-a-very-long-really-very-long-name-of-this-small-bdomain.server-daten.de
And saw the error @mnordhoff has described:
{
"type": "urn:ietf:params:acme:error:malformed",
"detail": "Error finalizing order :: CN was longer than 64 bytes",
"status": 400
}