MirrorManager is a piece of SW Fedora/RedHat utilizes to provide yum with the lists of mirrors to yum. I.e., “MirrorManager not working correctly”, in this case means yum on a users’s system receives incorrect data.
On the user side, one way to work around the effects of this defect is to remove the metadata files your yum caches.
E.g. by using “yum clean < ...all,metadata>” or by brute force: # rm -f /var/cache/yum/updates/{*xml*,*sql*,*cookie} and to rerun “yum update”.