This error message is displayed by simplesaml.php v1.13.2.
The Pac4j SAML lib has been upgraded from encrypting only in SHA-1 to encrypting SHA-256 signatures and SHA-512 digest. The SHA-512 digest method is poorly supported by this version of simplesaml.
To fix this issue, simplesaml.php must be updated to at least v1.14.x.
The IdP must be configured to accept SHA-256 and SHA-512 encryption methods.