Here are some more tips related to the usage of CellsHelper.FontDirs property. Firstly, the following statements should be at the start of your application, that is; before initializing an object of the Workbook class.
string fontDir = GetFontDirectory();<o:p></o:p>
if (fontDir != null)
Workbook srcbook = new Workbook("sample.xlsx");
Secondly, could you please try the scenario by changing the method to specify the font directory? For instance, you may alternatively use the property CellsHelper.FontDir that accepts one string value.