X_PWA_VAPID_KEY_INVALID
Means
the VAPID key pair is malformed or not a pair
Typical cause
ULTIMATE_VAPID_PUBLIC_KEY is not a 65-byte uncompressed P-256 point, ULTIMATE_VAPID_PRIVATE_KEY is not a 32-byte scalar, or the private key does not sign for the public one (the boot signs a probe and verifies it)
Fix
x vapid create — one command writes both halves, so they cannot disagree