It seems like the font choice is somehow embedded in the document class itself; I've tried choosing fonts the usual way, but with no effect.
This works for me:
The problem might be that you loaded a font package that doesn't provide a sans-serif font, e.g.
This makes the serif font the default font. The standard for
The class loads the (obsolete) package
If you want to use, say, the Computer Modern fonts, you have to revert the setting:
Alternatively, you can load a different font package:
or any font package of your choice.