Okay, i just wanted to make sure the additional server is a DC backup. During normal operation the App Center will install these packages on available backup servers, but you just added a new one… To install the packages on your DC Backup try.
univention-app update; univention-app install --only-master-packages kopano-core
If you do not have the most recent kopano-core app version on your master, you have to append the app version installed on the DC master to the univention-app install command, e.g. kopano-core=8.6.8.2