Started working today and Chromium on Ubuntu suddenly throws

NET::ERR_CERTIFICATE_TRANSPARENCY_REQUIRED on a well known host.

Already tried deleting .pki/nssdb/*, but nothing changed.

I have the same problem. This is what I found out so far:

https://bugs.launchpad.net/ubuntu/+source/chromium-browser/+bug/1641380 https://bugs.chromium.org/p/chromium/issues/detail?id=664177 https://knowledge.symantec.com/support/ssl-certificates-support/index?page=content&id=ALERT2160

Are you also using Version 53.0.2785.143?

Looks like it is important when the build happened: https://chromium.googlesource.com/chromium/src/net/+/master/cert/ct_policy_enforcer.cc#44

Looks like the best workaround is to upgrade to 54.x, wait for a patch or use another browser.

Update: A new version of chromium, that fixes the problem, was just released (at least for ubuntu).