Uwe Stöhr wrote: > The reason is that MiKTeX loads marvosym automatically and I currently > don't know why. Could you please report this on > http://bugzilla.lyx.org
It's not MikTeX. This is a well known g-brief bug (it relies on marvosym). The g-brief maintainer is informed about this, but I'm not sure he cares. We also already have a bugzilla report somewhere. Jürgen