A colleague found out, that the certificate on the ADFS server has a automatic rollover and per default is valid for 1 year.
But after the rollover of the key the SAML authentication does not work anymore for IBM Notes clients. There´s a quite simple solution
>> “Re-Download” the “FederationMetadata.xml” file from the ADFS server and reimport this file in >> the IdPCat configuration.
Afterwards SAML authentication is working fine again.