Entering the IP-Addresses of those domains to the whitelist did not help:
outbound1.letsencrypt.org and outbound2.letsencrypt.org
Still: Valid Until 1970-01-01
Allowing ALL Countries in GEO-Firewall helped.
Valid Until 2020-05-20
Enabeling Geo-Firewall again: Valid Until 1970-01-01
