Let's encrypt failed - multiple subdomains on different servers

Hi all,
All my sites that are on this webserver are running fine with Let'sencrypt apart from one.
That particular domain has its mail.domain.com to a different IP (non-ispconfig server) along with a few other subdomains.
The main web site is under ispconfig but doesn't pass the certification in the browser.
Under logs I can't see a problem?

Code:

root@web1:/var/www# tail -f /var/log/letsencrypt/letsencrypt.log
2020-09-25 22:51:03,941:DEBUG:certbot._internal.main:certbot version: 1.8.0
2020-09-25 22:51:03,941:DEBUG:certbot._internal.main:Arguments: ['--domains', 'mydomain.com']
2020-09-25 22:51:03,942:DEBUG:certbot._internal.main:Discovered plugins: PluginsRegistry(PluginEntryPoint#apache,PluginEntryPoint#manual,PluginEntryPoint#nginx,PluginEntryPoint#null,PluginEntryPoint#standalone,PluginEntryPoint#webroot)
2020-09-25 22:51:03,959:DEBUG:certbot._internal.log:Root logging level set at 20
2020-09-25 22:51:03,960:INFO:certbot._internal.log:Saving debug log to /var/log/letsencrypt/letsencrypt.log
2020-09-25 22:51:03,978:DEBUG:urllib3.connectionpool:Starting new HTTP connection (1): ocsp.int-x3.letsencrypt.org:80
2020-09-25 22:51:03,991:DEBUG:urllib3.connectionpool:http://ocsp.int-x3.letsencrypt.org:80 "POST / HTTP/1.1" 200 527
2020-09-25 22:51:03,993:DEBUG:certbot.ocsp:OCSP response for certificate /etc/letsencrypt/live/mydomain.com/cert.pem is signed by the certificate's issuer.
2020-09-25 22:51:03,995:DEBUG:certbot.ocsp:OCSP certificate status for /etc/letsencrypt/live/mydomain.com/cert.pem is: OCSPCertStatus.GOOD

 

Enabling the option "Skip Letsencrypt check" under System > Server config > web make it worked.