I managed to get it running. Luckily I had backups of the VMs so I could restart from the beginning several times. There are two things I did differently now when it worked.
Firstly I managed to remove that broken zeppelin service which always bugged me.
Secondly, well, for registration of the new HDP Version, I set up Ambari to use a proxy so it could fetch the repository URLs. This wasn't so great. I noticed that, while making it possible for ambari to reach the www, no metrics were available anymore and none of the views (HIVE, TEZ, HDFS) worked. I always got response code 503 iirc. I could still start and stop services, though. Probably the proxy blocked some of the ports? Anyway, so I removed the proxy settings after the installation of the new HDP version and before upgrading to it.
If I were to bet, I'd say the proxy is to blame... maybe you have connection problems, too?