Hey there,
I’m updating Openhab on Ubuntu 18.04.
While running apt-get update this message appeared:
Err:3 https://dl.bintray.com/openhab/apt-repo2 stable Release
Certificate verification failed: The certificate is NOT trusted. The name in the certificate does not match the expected. Could not handshake: Error in the certificate verification.
After a debugging for a while I found out that apt-cacher-ng is trying to connect via IPv6 to bintray. It turns out bintray isn’t supporting IPv6. They have only legacy IPv4 support 
For now I’ve disabled apt-cacher-ng for the openhab host. This is only a workaround until I can get my packages via IPv6
Thanks,
Florian