Thanks. That was more than enough help. It worked.
(Added following line to the fonts.xml like suggested:
<TextStyle Algorithm="Simple" FontName="Zapf Calligraphic 801 Bold BT" Language="english" Name="Font_Tooltip" PointSize="15" PrivateFontPath="UI/Fonts/zcal801b.TTF" UseCachedFont="true"/>
...it's working well again.)