Probably because MS update 2661254 has broken connecting to small privatekey sizes. For each yubikey RADIUS server you should do the following: Create a new PEM file with a 2048 bit key size – eg yubi.pem Copy the PEM file to /etc/webmin on each yubi box edit /etc/webmin/miniserv.conf set keyfile=/etc/webmin/yubi.pem /etc/init.d/webmin restart