RSA Signature Authentication problem using XCA

Hello,

I have a site to site vpn set up with m0n0 to m0n0 using pre shared keys 
and it has been working for over a year.

I would like to use RSA Signature Authentication and have followed the 
following instructions:
<http://doc.m0n0.ch/handbook/ipsec-rsa-auth.html>

When I try to "Save" the configuration on the m0n0wall web interface on 
the ipsec edit page, I get the following:

The following input errors were detected:

    * This key does not appear to be valid.

I think this is referring to the private key.
I have searched the mailing list, the forum, and the web, but I don't 
see anything that even comes close to explaining this.

When I open "XCA" and look at the private key details, it says "Private 
Exponent: Available"

When I look at the corresponding certificate details, it says "Validity: 
Valid".

m0n0wall is great.
Any help is greatly appreciated.

Joe