percona cluster 5.6 - handshake with remote endpoint failed, but endpoint is not part of config
I feel a bit stupid, but the answer to that strange behavior was simple:

The old node1 cluster node was trying to connect to the cluster again after a reboot. I forgot to uninstall the mysql installation on that node and instead only stopped the mysql server. That was well as long the system did not restart mysql.

To fix the situation I removed the old config files on node1 and also uninstalled the percona cluster packages.

Everything is peachy now.