Internet Access after renew the expired com.apple.servermgrd certificate
The com.apple.servermgrd certificate is only used to create a SSL connection from Server.App to the server backend admin daemon. It is safe to delete as it will be recreated. You should delete the cert with the following commands:

sudo security set-identity-preference -n -s com.apple.servermgrd /Library/Keychains/System.keychain
sudo security delete-certificate -c com.apple.servermgrd /Library/Keychains/System.keychain
sudo killall servermgrd 


For Mojave and later: 

sudo killall servermgr-listener


This will delete the certificate &amp; the identity preference and restart the backend daemon, which should happily recreate the com.apple.servermgrd certificate. The other SSL using server services default to using a certificate that has the machine's DNS name.