Hi Filip,
We have the same issue here after upgrading from v12.1.2 to v13.1.
It seems that google site is sending in his chain of certificates a root CA for Geotrust that was signed by "Equifax Secure Certificate Authority" but it is not sending the CA cert.
Looking into my box ca store /config/ssl/ssl.crt/ca-bundle.crt I can see "Equifax Secure Certificate Authority" was removed on the ca-bundle in v13.1 (it expires on August 2018).
I have fixed the issue by manually adding this root CA cert to the trusted store. In my lab this is the process
no reboot require to fix the issue.
Hope it helps on your issue too.
Best Regards
David Martín