alp on Nostr: I can only guess, but the most common cause would be that the relay in the bunker:// ...

I can only guess, but the most common cause would be that the relay in the bunker:// token isn't reachable or isn't responding to kind 24133. Amber outputs the token with a relay=… parameter. If this relay is offline, slow, or has restrictive filters, you get exactly this error. You could try to regenerate the bunker token in Amber using a reliable NIP-46 relay like wss://relay.nsec.app.