Im seeing the same issue on my current win7 install, but installing 766 on a new win10 machine has no trouble

both of them give same reply to that as

echo ## Certificate data from Mozilla as of: Tue Jun 29 09:58:15 2021 GMT

and both have same

//echo -a $sha256(cacert.pem,2) is 1e8aec6afd4e62b3bb60f2f0ee658c5e528c88c28c160525e068edc1645e135e

Someone in channel has the problem on his existing win10 setup. He just did a fresh portable 766, and still same popup warning from the libera.chat servers. In the network support channel, nobody else using a different client than mirc is reporting this.