One of the TLS/SSL certificates used by your server uses a key that is considered weak due to its small key size. The recommended minimum sizes for RSA and ECDSA keys are 2,048 bit and 256 bit, respectively.
This alert is not necessarily caused by the server (leaf) certificate, but may have been triggered by an intermediate certificate. Please refer to the certificate serial number in the alert details to identify the affected certificate.
If you are using certificates with weak keys, you will need to migrate to either larger keys, more efficient algorithms, or both.
Moving to a 2048-bit certificate