Cannot upgrade from Fedora 23 to Fedora 24

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Actually I wanted to upgrade from Fedora 23 directly to 27, which failed as well as an direct upgrade to 25 which also failed. Hence I thought I do the upgrade step by step and upgrade my Fedora 23 to Fedora 24. However this update was also not successful due to a warning which is visble in the log down below.

My Question: How can I resolve this warning that prevents my upgrade from F23 to F24?

I mainly followed this instruction

After executing sudo dnf system-upgrade download --refresh --releasever=24 --allowerasing a long lust of packages is shown and (seemingly downloaded) but at the end, I get a warning: After the warning I also executed a check for the installed version and it still indicates Fedora 23.

Here is the log from the terminal:

 (867/867): webkitgtk-2.4.11-2.fc24.x86_64.rpm                                          6.2 MB/s |  12 MB     00:01                                                                                                                                                            
   ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
    Total                                                                                                                                                                                                         17 MB/s | 516 MB     00:29
    warning: /var/lib/dnf/system-upgrade/python2-iniparse-0.4-19.fc24.noarch.rpm: Header V3 RSA/SHA256 Signature, key ID 81b46521: NOKEY
    Importing GPG key 0x81B46521:
     Userid     : "Fedora (24) <fedora-24-primary@fedoraproject.org>"
     Fingerprint: 5048 BDBB A5E7 76E5 47B0 9CCC 73BD E983 81B4 6521
     From       : /etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-24-x86_64
    Is this ok [y/N]: y
    Key imported successfully
    warning: /var/lib/dnf/system-upgrade/mysql-community-libs-5.6.38-2.el7.x86_64.rpm: Header V3 DSA/SHA1 Signature, key ID 5072e1f5: NOKEY
    Empty mirrorlist and no basepath specified!
    The downloaded packages were saved in cache until the next successful transaction.
    You can remove cached packages by executing 'dnf clean packages'.
    [root@sgbp7z ~]# cat /etc/*release
    Fedora release 23 (Twenty Three)
    NAME=Fedora
    VERSION="23 (Twenty Three)"
    ID=fedora
    VERSION_ID=23
    PRETTY_NAME="Fedora 23 (Twenty Three)"
    ANSI_COLOR="0;34"
    CPE_NAME="cpe:/o:fedoraproject:fedora:23"
    HOME_URL="https://fedoraproject.org/"
    BUG_REPORT_URL="https://bugzilla.redhat.com/"
    REDHAT_BUGZILLA_PRODUCT="Fedora"
    REDHAT_BUGZILLA_PRODUCT_VERSION=23
    REDHAT_SUPPORT_PRODUCT="Fedora"
    REDHAT_SUPPORT_PRODUCT_VERSION=23
    PRIVACY_POLICY_URL=https://fedoraproject.org/wiki/Legal:PrivacyPolicy
    Fedora release 23 (Twenty Three)
    Fedora release 23 (Twenty Three)