It seems that my problem is fixed with your suggestions.
I added this to the Cinder.conf:
#logdirabbitMQ broker address where a single node is used. (string value)
# The RabbitMQ broker port where a single node is used. (integer value)
# RabbitMQ HA cluster host:port pairs. (list value)
# Connect over SSL for RabbitMQ. (boolean value)
# The RabbitMQ userid. (string value)
# The RabbitMQ password. (string value)
# the RabbitMQ login method (string value)
# The RabbitMQ virtual host. (string value)
# How frequently to retry connecting with RabbitMQ. (integer value)
# How long to backoff for between retries when connecting to RabbitMQ. (integer value)
# Maximum number of RabbitMQ connection retries. Default is 0 (infinite retry count). (integer value)
# Use HA queues in RabbitMQ (x-ha-policy: all). If you change this option, you must wipe the RabbitMQ database. (boolean value)
# If passed, use a fake RabbitMQ provider. (boolean value)
I also did thesame for nova. And I don't get any error messages anymore.
But for some reason I also don't get a "Connected" log entry. is it true that Nova or Cinder does not report a connected state? I rembered that I once read it somewhere that there should be some sort of "Connected to...." log entry.