This is happening because you're sending the entire private key in the embedded JWK.
[ ACME ] > b64url -decode
Input data to encode/decode. End by sending '.'
[ ACME ] > b64url data > eyJhbGciOiJFUzI1NiIsImp3ayI6eyJjcnYiOiJQLTI1NiIsImQiOiJSR05QSXZ4WHRFTEl3QjdrR3A4SFRfc0NNUHk5akZNb3Jwb1Q3SlRPVVRJIiwia2lkIjoiQkMxNTJHZUItZlFLZnpmUDI0Sk9rbDlvZVhOVFBRbGhtM2pZaW5UR2RlUSIsImt0eSI6IkVDIiwieCI6ImFvMlZMVXBvV2UzT054bmRpUFpVTjdhS1NGQzFLYS02a254TnN2d2hYYkUiLCJ5IjoiRFVkOXgzSUtEaHVBdWMyVl8xdXRfOFM0MV9kVlhzUUdTdHBDanVFVkNCSSJ9LCJub25jZSI6IkpLd3lvcnlySFpfYnRtMTBUdVFtTnciLCJ1cmwiOiAiaHR0cHM6Ly8xNzIuMTkuMTkxLjIzNjoxNDAwMC9zaWduLW1lLXVwIn0
... .
Result:
{"alg":"ES256","jwk":{"crv":"P-256","d":"RGNPIvxXtELIwB7kGp8HT_sCMPy9jFMorpoT7JTOUTI","kid":"BC152GeB-fQKfzfP24JOkl9oeXNTPQlhm3jYinTGdeQ","kty":"EC","x":"ao2VLUpoWe3ONxndiPZUN7aKSFC1Ka-6knxNsvwhXbE","y":"DUd9x3IKDhuAuc2V_1ut_8S41_dVXsQGStpCjuEVCBI"},"nonce":"JKwyoryrHZ_btm10TuQmNw","url": "https://172.19.191.236:14000/sign-me-up"}
See the d parameter? That's the private part of an ECDSA key. You don't want to send that to an ACME server, that's a security blunder that Pebble is stopping you from committing. (Though Pebble should definitely make a much less opaque error).