Hello Vladimír,

Please correct me if I am wrong but from what I understand you are talking about having an Author for the actual document. This piece of metadata is supported by the library and can be obtained through the DocumentInfo property of the Workbook: 

Workbook workbook;
XlsxFormatProvider xlsxFormatProvider = new XlsxFormatProvider();

using (Stream input = new FileStream("inputFile.xlsx", FileMode.Open))
{
    workbook = xlsxFormatProvider.Import(input);
}

var author = workbook.DocumentInfo.Author;

...however, it does not play a role in this logged issue. This means that your document can have an author or not have an author and it will not cause any unusual results. 

Yet, this bug concerns the missing 'Author' property of Comments instead.

With that in mind, please verify whether or not the document you are using contains such comments and if so, please know that the exception you are experiencing is expected. However, if the file does not contain comments with no author, please feel free to open a separate ticket regarding your scenario so we can continue our conversation in private and try to resolve the issue you are facing there.

Regards,
Yoan
Progress Telerik