Re-initialize etcd-cluster completely (on coreos)
I managed to fix it. First I disabled and deleted all etcd2-related units. Then I created a new dropin to the etcd-member.service with the following content:

[Service]
Environment="ETCD_OPTS=--name cluster00 \
  --discovery https://discovery.etcd.io/&lt;MY_ID&gt; \
  --listen-client-urls http://&lt;NODE_IP&gt;:2379,http://127.0.0.1:2379 \
  --listen-peer-urls http://&lt;NODE_IP&gt;:2380 \
  --advertise-client-urls http://&lt;NODE_IP&gt;:2379 \
  --initial-advertise-peer-urls http://&lt;NODE_IP&gt;:2380"


I generated a new discovery id for the cluster.

Then I teared down the old cluster completely, by deleting all content of /var/lib/etcd/* and the directory /var/lib/etcd2 completely and (re-)started the etcd-member service.

Be aware, that I don't have ssl enabled at this point!