Let's encrypt issues

[steven]

Ok, I found a solution,

I set up a separate VM with certbot installed and requested new certificates for my domain.
This assumes that you have a DNS record publicly set up. I use Cloudflare for this.

On Debian/Ubuntu system use:

sudo apt install certbot

Then request the certificates for you domain (example.com << use your domain here)

sudo certbot --manual --preferred-challenges dns certonly -d example.com

Follow the instructions. Make sure to create a new TXT record in Cloudflare when prompted during the certbot request:

_acme-challenge.example.com

The new certificates can be found in the

/etc/letsencrypt/archive/example.com/cert1.pem
/etc/letsencrypt/archive/example.com/chain1.pem
/etc/letsencrypt/archive/example.com/fullchain1.pem
/etc/letsencrypt/archive/example.com/privkey1.pem

I copied the fullchain1.pem and privkey1.pem to a custom directory I created on my Univention server:

/etc/myssl/fullchain1.pem
/etc/myssl/privkey1.pem

Please note that the certificates will be renewed automatically but not copied to the Univention server. Perhaps a script can automate this process. Should I find a solution then I will update this post. If someone else has a better solution or a script please feel free to post it here

With the certificates in place I followed this forum post:

Using your own SSL certificates

By default, certificates created by the UCS CA are used in UCS for the Apache, Postfix, Dovecot (or Cyrus) etc. daemons. As described in the following points, the Apache, Dovecot, Cyrus and Postfix services can be configured for the use of externally created certificates. The actions must be done on the server the service is running: Note: In case you have no `.pem file for the below services you can convert them through the following: Say you have ca_bundle.crt my_de.crt my_de.key convert …

Ensure you change the name of your certificate if needed.

Dovecot

Dovecot is the default IMAP server since UCS 4.0-2 and supersedes Cyrus.

The UCR variables mail/dovecot/ssl/certificate and mail/dovecot/ssl/key must be set for Dovecot:

ucr set mail/dovecot/ssl/certificate="/etc/myssl/fullchain1.pem"
ucr set mail/dovecot/ssl/key="/etc/myssl/privkey1.pem"

Then restart the daemon:
service dovecot restart

Postfix:

The UCR variables mail/postfix/ssl/certificate and mail/postfix/ssl/key need to be configured:

ucr set mail/postfix/ssl/certificate="/etc/myssl/fullchain1.pem"
ucr set mail/postfix/ssl/key="/etc/myssl/privkey1.pem"

Then the mail server has to be restarted:
service postfix restart

I want to thank everyone for your help and I hope this guide helps others which are in the same situation where http-01 challenges are not an option.
I hope Univention will add DNS challenges in the future.