It appears this is a general error encountered by users who try to sign their PDF files. The apps used are totally different than the one you already have which means there must be a problem with the Windows installation.
I recommend applying all the Windows Updates available through your operating system in Windows Update.
Additionally, you could reset the Cryptographic Service. To do this, go to Start > Run > services.msc. Right click the service in the list and then select Restart.