RHEL 6.4 to 6.5: GPG keys not correct?

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Dear all,

I'm trying to update a RHEL 6.4 system to 6.5 using "yum update", but get an error:

[root@srv-linux01 ~]# yum update

[... ]

Transaction Summary
Install      17 Package(s)
Upgrade     410 Package(s)
Remove        2 Package(s)

Total download size: 645 M
Is this ok [y/N]: y


Total                                           5.0 MB/s | 645 MB     02:10     
warning: rpmts_HdrFromFdno: Header V3 RSA/SHA256 Signature, key ID f21541eb: NOKEY
Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release

The GPG keys listed for the "Red Hat Enterprise Linux 6 Server (RPMs)" repository are already installed but they are not correct for this package.
Check that the correct key URLs are configured for this repository.

I've added a few non-RHEL repositories (adobe-linux, epel, rpmfusion) and enabled a redhat.repo entry (rhel-6-server-cf-tools-1-rpms). Minor upgrades (6.2 -> 6.3 -> 6.4) always went smoothly.

However, I see old redhat-repo files in /etc/yum.repos.d/. One from Feb 28 2012 with different "sslclientkey" and "sslclientcert" entries, and one from Feb 25 2013 without "ui_repoid_vars = releasever basearch" lines. Experimenting with these files, however, yielded the same error.

I would be very glad with some help.