yum update on CentOS complains about "Multilib version problems" of "nss-softokn-freebl" - yum

Question

On last Friday morning, I tried a "yum update" on my CentOS laptop, and it reported this:
Loaded plugins: fastestmirror, langpacks, verify
Loading mirror speeds from cached hostfile
* base: repo1.dal.innoscale.net
* epel: fedora-epel.mirror.lstn.net
* extras: mirror.unl.edu
* nux-dextop: mirror.li.nux.ro
* rpmfusion-free-updates: mirror.us.leaseweb.net
* updates: mirror.spro.net
Resolving Dependencies
--> Running transaction check
---> Package nss-softokn-freebl.i686 0:3.16.2.3-13.el7_1 will be updated
---> Package nss-softokn-freebl.i686 0:3.16.2.3-14.2.el7_2 will be an update
---> Package python-ecdsa.noarch 0:0.11-3.el7.centos will be obsoleted
---> Package python2-ecdsa.noarch 0:0.13-4.el7 will be obsoleting
---> Package tzdata.noarch 0:2016c-1.el7 will be updated
---> Package tzdata.noarch 0:2016d-1.el7 will be an update
---> Package tzdata-java.noarch 0:2016c-1.el7 will be updated
---> Package tzdata-java.noarch 0:2016d-1.el7 will be an update
--> Finished Dependency Resolution
Error: Multilib version problems found. This often means that the root
cause is something else and multilib version checking is just
pointing out that there is a problem. Eg.:
1. You have an upgrade for nss-softokn-freebl which is missing some
dependency that another package requires. Yum is trying to
solve this by installing an older version of nss-softokn-freebl of the
different architecture. If you exclude the bad architecture
yum will tell you what the root cause is (which package
requires what). You can try redoing the upgrade with
--exclude nss-softokn-freebl.otherarch ... this should give you an error
message showing the root cause of the problem.
2. You have multiple architectures of nss-softokn-freebl installed, but
yum can only see an upgrade for one of those architectures.
If you don't want/need both architectures anymore then you
can remove the one with the missing update and everything
will work.
3. You have duplicate versions of nss-softokn-freebl installed already.
You can use "yum check" to get yum show these errors.
...you can also use --setopt=protected_multilib=false to remove
this checking, however this is almost never the correct thing to
do as something else is very likely to go wrong (often causing
much more problems).
Protected multilib versions: nss-softokn-freebl-3.16.2.3-14.2.el7_2.i686 != nss-softokn-freebl-3.16.2.3-13.el7_1.x86_64
I asked about this on #centos, and someone gave me some exploratory advice, but no real solution.

Answer 1

I experienced the very same issue on a Fedora 20 system (run in a docker container) when trying to install i686 development libraries. Reason were two not matching versions for x86_64 and i686, respectively.
Protected multilib versions: nss-softokn-freebl-3.19.1-1.0.fc20.i686 != nss-softokn-freebl-3.19.2-1.0.fc20.x86_64
For me it helped to call
yum distribution-synchronization
That automatically downgraded the x86_64 version. After that
I could install with
yum install nss-softokn-freebl.i686
and
yum list installed | grep nss-softokn-freebl
showed now:
nss-softokn-freebl.i686 3.19.1-1.0.fc20 #updates
nss-softokn-freebl.x86_64 3.19.1-1.0.fc20 #updates
Problem solved.