In some cases signing errors occur when using a PKCS #11 provider key for authentication. If your PKCS #11 provider (for example a hardware token) has multiple keys, it is possible that not all the keys can be used for authentication.
Try changing the Slots value in the PKCS #11 configuration (see Section PKCS #11 Provider).
When experimenting with the value, saving the settings and restarting the application, you will see different keys being used for authentication. Upload the keys one at a time to the remote host computer. One of the keys may be valid for authentication.