Final encryption step failing to enable "OK" box once certificate selected

I am using the GpgOL add-in for Outlook 2010 on a Windows 7 machine. In trying to encrypt an email message (in an identical manner as I have donebefore). I click Encrypt message in the Plug in, I then get a pop-up called"Select Certificate for Message GpgOL", and I click OpenPGP and the certificate of the recipient shows up on the drop down. I select the certificate but the “OK” button on this pop-up remains grayed out so it does not proceed with the encryption. So my only option in “Cancel”. What could be causing this? I did this exact procedure a week or so ago, and it worked fine.

Just an idea: The certificate may have been expired in the meantime, so it is not trusted anymore. You could try to to check the certificate details. If this does not work, try to encrypt a file to the same recipient.

I gave up and switched to Symantec desktop email encryption product which is a joy to use in comparison.

Sorry about this. Thanks for trying Gpg4win in the first place.
Recommendations about what we could improve area always welcome.