Dear RG305.
Thank you for all your help. Problem solved. Basically, all steps described here
https://wiki.zimbra.com/wiki/Installing_a_LetsEncrypt_SSL_Certificate
The exact step, which resolved was
wget -O /tmp/ISRG-X1.pem https://letsencrypt.org/certs/isrgrootx1.pem.txt
cat /tmp/ISRG-X1.pem >> /etc/letsencrypt/live/barrydegraaff.tk/chain.pem