Links to admin panel by hostname don't work, only by IP

Hi

I cant access the admin panel by addressing the hostname. Only IP-links are working. But when I open the Hostname one port 80 I a HTTP Server Test page.
I've got a this new problem since two days and can't check my e-mails anymore. DNS records look correct for me, the hostname subdomain points to the IP, rDNS/PTR-check says SUCESS.
Do I have to generate some new files in /etc/pki/..? Please help.
Thanks

when you hit the change hostname button under hostname module it will generate ssl for it.
Ensure you didn't have added the server hostname as subdomain/domain

Hi Sandeep

That worked after I deleted the server hostname as subdamain. Thank you. But now I can't enable the firewall anymore and I can't connect to the mailserver running on this host. Roundcube says no connection to IMAP-Server.
Could this problem be related to 100% quotas on several /dev/loop(x) --> (/var/lib/snapd/snap/core/..  /var/lib/snapd/snap/certbot/..)?
I did never mention these 100% quotas before.

You can ignore the /dev/loopXX, that is okay