Server: VPS Centos 6.8 32-bit
Webmin 1.870 and 1.880 - OpenVPN+CA does not launch properly. Modifying the URL by removing 'navigation....' from it will allow launch however any attempt at certificate authority creation and attempt generate certificates against existing CA will result in failed operation and failure message. Also, when launched in this manner the screen produces a black background with white text which is inverse to usual display characteristics.
Which Webmin module are you seeing this in?