When you encounter the following error:
** Creating file '/opt/zimbra/ssl/zimbra/jetty.pkcs12'
ERROR: openssl pkcs12 export to '/opt/zimbra/ssl/zimbra/jetty.pkcs12' failed(1):
unable to load certificates
140665143981720:error:0906D066:PEM routines:PEM_read_bio:bad end line:pem_lib.c:805:
Do this (when for example having all the certs temporarily stored in /tmp/):
chown zimbra:zimbra /tmp/*.crt chmod 666 /tmp/*.crt