I haven't seen this error before, but my guess is that the problem lies in the installation of the Payflow Pro software. It uses OpenSSL for authentication, so it has to be able to find that, and then OpenSSL needs to be able to find your secure certificate. So there are several places where this could go wrong.
Unfortunately it has been a long time since I last installed this and the current clients I have using Verisign are former Cybercash clients so we are still using the old Cybercash code (works just fine, but probably would not work for a new Verisign account). So I don't have any examples of how to set it up correctly.
Hopefully someone who has used it recently will jump in here. However, you should know that in the upcoming release of OpenACS we've deprecated our PayflowPro support, due to the issues discussed in this thread:
https://openacs.org/forums/message-view?message_id=93102
If you are using a reasonably current Linux distro you will have the same problems once you do get things set up right, so you might want to think about using Authorize.net instead anyway. If you are using some other OS then we will all be waiting to hear whether it works ok for you or not - that was not known at the time we deprecated it.
