I have updated the Webmin theme to version 1.82 today. This release addresses issues with running Webmin behind an Apache (or similar) proxy.
There were a few instances in the theme were absolute hyperlinks were used instead of referencing the Webmin subdirectory parameter. If you are not running Webmin behind a proxy then this update will not effect you, however as this update may resolve proxy and VirutalMin related issues it is probably worthwhile updating if you are experiencing the occasional weird error.
Download from here: http://www.stress-free.co.nz/sites/default/files/theme-stressfree.tar.gz