Well, figured it out myself, here’s how to solve it, if someone rans into it, too:
First, check the https-feature of apt with debug: option:
> apt-get update -o Debug::Acquire::https=1
In my case, there have been errors, like “no access to ca_certificates.crt” and so on.
The read/write-permissions of /etc/ssl/certs/ were set propperly, so i did reinstall the certificate_ca and apt-transport-https:
> apt install ca-certificates apt-transport-https
Verifying, with the debug outpout again to see if it got fixed:
> apt-get update -o Debug::Acquire::https=1
The errors are gone, doing an apt-get update
> apt-get update
[...]
OK:7 http://ftp.de.debian.org/debian stretch Release
Ign:8 https://download.kopano.io/supported/core:/final/Debian_9.0 InRelease
Ign:2 https://attic.owncloud.com/org/download/repositories/production/Debian_9.0 InRelease
OK:9 http://download.owncloud.org/download/repositories/production/Debian_9.0 Release
Ign:10 https://download.kopano.io/supported/webapp:/final/Debian_9.0 InRelease
OK:11 https://download.kopano.io/supported/core:/final/Debian_9.0 Release
OK:12 https://download.kopano.io/supported/webapp:/final/Debian_9.0 Release
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