DKIM time out and mail SSL certificate problem v12

SvenK

Hi,
I recently added a second mail server to my cluster. Then I made the update to v12 (don't know if this has someting to do with my problem).
Now I added a new project and selected the new mail server for the first time.
Unfortunately I have to problems now:

I can't activate DKIM - it's always running into a time out.
When I add one of the mail accounts to Apple Mail, I get the message "SSL certificate unsecure" (when I accept it anyway, it works). I tried to generate a new SSL certificate with the new "Generate Mail SSL" function from cpfence, but get an error message "Unknown error occurred".

Appart from these two problems, everything seems to work: Mails are comming in and going out. DNS status is green.
I don't know, if these two problems are connected and/or if this has to do with Enhance v12.

Any idea, how to fix these problems?
I would be grateful for any tips!

Btw.: On my older mailserver everything works fine.

CU...Sven

Adam

Does the new mail server have an external firewall? If so, can you make sure it allows all ports from the control panel or, or at least port 50000 TCP?

SvenK

Adam Yes, the mail server has an external firewall, but the ports between all my servers in my cluster are open (TCP and ICMP) and the other mail server uses the same firewall rules.
I added now port 50000 TCP, but the problem still remains, even when I disable the firewall completely.

Meanwhile I found out, that you can request a Let's Encrypt certificate specifically for mail in the control panel as well, but unfortunatelly it always throws an error: "Failed to issue Let's Encrypt certificate for 'mail.wildermuth-gymnasium.de".
Strangely enough requesting a Let's Encrypt certificate for the main domain works perfectly fine.

Any ideas?