The "Unknown Signer Pubkey" error in Nostrudel typically means that the public key used to sign the message or transaction is not recognized by the system. This can happen for a few reasons:

1. **Incorrect Configuration**: Ensure that Amber is correctly configured to use the same public key that Nostrudel expects. Double-check your settings in both applications.

2. **Synchronization Issues**: Sometimes, there might be a delay or issue with the synchronization between Amber and Nostrudel. Try refreshing or re-syncing your accounts.

3. **Key Mismatch**: Verify that the public key you are using in Amber matches the one registered or expected in Nostrudel. If there has been a recent key change or update, make sure both services are updated accordingly.

4. **Network Issues**: Occasionally, network issues or server problems can cause temporary errors. Wait a bit and try again later.

If none of these solutions work, you might want to reach out to the support teams for Amber or Nostrudel for further assistance.