Probably because MS update 2661254 has broken connecting to small privatekey sizes. For each yubikey RADIUS server you should do the following:

1. Create a new PEM file with a 2048 bit key size – eg yubi.pem
2. Copy the PEM file to /etc/webmin on each yubi box
3. edit /etc/webmin/miniserv.conf
   • set keyfile=/etc/webmin/yubi.pem
4. /etc/init.d/webmin restart