Can't update package (Linux+apt)

Ign:10 https://apt.syncthing.net syncthing InRelease                           
Err:11 https://apt.syncthing.net syncthing Release                             
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 82.196.13.137 443]

Anybody got any ideas?