That is weird, I can't get much information about this error anywhere. What Windows version has the machine with the problem? Is it running as a non admin user? Can you manually access c:\Windows\fonts with the windows explorer?
On our side, I've modified the error message so it tells you which folder is failing, but I am not sure we can do much more about it. FlexCel needs access to the Fonts folder to be able to export to pdf, and if Windows is throwing an exception when you try to get the font folder, there is not much we can do about it.
As a workaround you can use the OnGetFontFolder event to redirect FlexCel to use an specific folder that you know exists in the machine, or you can copy the fonts inside a "Fonts" folder immediately below your exe and FlexCel will read them from there. But you should be able to access the font folder in the machine.