It looks to me like this is the important part - the ownership of a file is wrong, somehow:
In my case (a working installation not updated for a week or so) this file is owned by root:
# cd /var/discourse/
# ./launcher enter app
x86_64 arch detected.
/var/www/discourse# ls -l /etc/ssl/private/ssl-cert-snakeoil.key
-rw-r----- 1 root ssl-cert 1704 Jul 20 00:14 /etc/ssl/private/ssl-cert-snakeoil.key
# exit