I noticed the patches were available according to XO on the recently installed host, but not on the host I am using already for a longer time (and used to install the test-patches before). This host is also the pool master.
Only after a yum clean all and a yum update the updates were visible on the first host and thus the pool.
After fixing that, the Rolling Pool Update went very smooth. I like this feature!
Thanks for the feedback. I think we must add a feature to do that from the plugin ("force refresh updates"). Pinging @nraynaud about this.