Hi dvshestakov,
Thank you for your interest in Syncfusion products.
I am not able to reproduce the issue that you mentioned here. As per your requirements I have created a file, protect the workbook(Tools->Protection->ProtectWorkbook) and open the file in XlsIO using OpenReadOnly or open method. It works without any exception.
Here is the sample for your reference:
http://www.syncfusion.com/development/uploads/XlsIO_Win_OpenReadonly_aad2fee4.zipPlease take a look into the above sample and if still the issue exists, could you please try reproducing it in the above sample and send us the modified sample with your file so that we could sort out the cause of the issue and provide you a solution?
Also please provide details about the system configuration and Essential studio version that you have used.
Regards,
G.Yavana