This issue typically happens for a 2-Way TLS, when the certificate sent by the client is expired. In a 2-way TLS, both client and server exchange their public certificates to accomplish the handshake. The client validates the server certificate and the server validates the client certificate. During the TLS handshake if it is found that the client certificate is expired, then the server will send 400 Bad request with the message “The SSL certificate error”. The solution for this problem is that procure a new certificate and upload the certificate.

https://net-informations.com/q/mis/400.html