dear readers, may be this is not lyx but TeX error. (I use the last Miktex WinXP)
mavrosym.sty (2006/05/11 v2.1) defines \def\Telefon{{\mvchr84}} while g-brief-de.layout (and the g-brief.cls in tex) defines Style Telefon ... LatexName Telefon ... Endthis stops latex every time using g-brief with errors when running automatic through mavrosym while using g-brief.
I would recommend for mavrosym.sty to change "Telefon" to "Telefone"! best regards Michael