I get
at Syncfusion.Pdf.Security.Asn1Sequence.get_Item(Int32 index)
at Syncfusion.Pdf.Security.PdfCmsSigner.Sign(Byte[] secondDigest, TimeStampServer server, Byte[] timeStampResponse, Byte[] ocsp, ICollection`1 crls, CryptographicStandard sigtype, String hashAlgorithm)
at Syncfusion.Pdf.Security.PdfSignatureDictionary.GetPortableStoreCertificate()
at Syncfusion.Pdf.Security.PdfSignatureDictionary.DocumentSaved(Object sender, DocumentSavedEventArgs e)
at Syncfusion.Pdf.PdfDocumentBase.OnDocumentSaved(DocumentSavedEventArgs args)
at Syncfusion.Pdf.Parsing.PdfLoadedDocument.Save(Stream stream)
when signing in parallel. In serial, one document after another, it works fine.