error trying to protect the worksheet

I use RadSpreadSheet to view xlsx stored in the database (so they need to be protected from editing). But for some worksheet, this leads to the following error:

System.ArgumentException: algorithmName
в Telerik.Windows.Documents.Utilities.Guard.ThrowExceptionIfNullOrEmpty(String param, String paramName)
в Telerik.Windows.Documents.Common.Model.Protection.ProtectionHelperBase.GenerateHashBase64(String password, String salt, String algorithmName, Int32 spinCount)
в Telerik.Windows.Documents.Spreadsheet.Model.Protection.ProtectionData.EnforceProtection(String input)
в Telerik.Windows.Documents.Spreadsheet.Model.Sheet.ProtectSheet(String password)

I cannot submit the file, as it contains confidential information. Perhaps the reason is that the worksheet contains links to other files that are not on this computer.

Here's a line from my program that causes an error:

tem.Protect("abc", WorksheetProtectionOptions.Default);