Error in the certificate verification

[Jörg-Volker Peetz <jvpeetz@web.de>]

Hi,

on Debian testing, when trying to update package lists with `apt update`, the 
following error message show up:

W: Failed to fetch https://deb.debian.org/debian/dists/testing/InRelease 
Certificate verification failed: The certificate is NOT trusted. The certificate 
chain uses expired certificate.  Could not handshake: Error in the certificate 
verification. [IP: 199.232.138.132 443]

The IP address varies between different calls.
Has anybody else encountered this problem?

Gpg tells that the signature of `InRelease` is made
    using RSA key 0146DC6D4A0B2914BDED34DB648ACFD622F3D138
and
    using RSA key A7236886F3CCCAAD148A27F80E98404D386FA1D9

Any idea anybody?

Regards,
Jörg.