dnf: Peers Certificate issuer is not recognized

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I can't do anything with dnf on a Fedora 25 system.

when I do dnf -v update I I get the following error:
cachedir: /var/cache/dnf
Loaded plugins: protectedpackages, download, copr, config-manager, generatecompletioncache, builddep, Query, playground, needs-restarting, noroot, debuginfo-install, reposync
DNF version: 1.1.10
Cannot download 'https://mirrors.fedoraproject.org/metalink?repo=fedora-25&arch=x86 64': Cannot prepare internal mirrorlist: Curl error (60): Peer certificate cannot be authenticated with given CA certificates for https://mirrors.fedoraproject.org/met... [Peer's Certificate issuer is not recognized.].
Error: Failed to synchronize cache for repo 'fedora'

I tried many things that did not help:
dnf clean all
dnf -v makecache
dnf update --refresh
dnf --refresh upgrade

Then I tried adding ... sslverify=0 ... the the config file and all sorts of wackiness ensued (revive Fedora ?!?) ... so I aborted and did not continue. Results below:

dnf --verbose install wkhtmltopdf
cachedir: /var/cache/dnf
Loaded plugins: Query, protectedpackages, builddep, debuginfo-install, copr, reposync, generatecompletioncache, config-manager, needs-restarting, download, noroot, playground
DNF version: 1.1.10
reviving: 'fedora' can be revived.
not found updateinfo for: Fedora 25 - x8664
reviving: failed for 'updates', mismatched sha256 sum.
Fedora 25 - x8664 - Updates 4.4 MB/s | 25 MB 00:05
fedora: using metadata from Tue Nov 15 13:49:18 2016.
updates: using metadata from Wed Oct 4 16:39:55 2017.
Last metadata expiration check: 0:00:06 ago on Thu Oct 5 15:31:31 2017.
Completion plugin: Generating completion cache...
--> Starting dependency resolution
---> Package wkhtmltopdf.x8664 0.12.3-2.fc24.1 will be installed

6 Oct Update:
Strange, yesterday I got this ...
dnf update
Error: Failed to synchronize cache for repo 'updates'
... and today I got this ...
dnf update
Error: Failed to synchronize cache for repo 'fedora'

How can I fix?