I quess the problem is that the simple Yubico OpenID provider puts our identity into the query string of your OpenID. Some consumers have problems with OpenID identities in the query string.
If you want to use OpenID and your Yubikey for Plaxo, you can do the following.
1. Create or configure a clavid OpenID account with either
1.1 If you don't have a clavid account yet, create one at
https://www.clavid.com/yubikey/start1.2 If you already have a claivd account, you can add your Yubikey in your Clavid profile
https://www.clavid.com/portal/2. Then you log-in to your plaxo account and add the claivd OpenID as an OpenID identity in your settings. The claivd openid looks like username.clavid.com
3. Then you can login using the OpenID username.clavid.com. On the clavid login page, you can choose 'YUBIKEY' as authentication type in the drop-down box.
Hope it helps. It worked for me.
PS: In addition, you can add multiple Yubikey's to your clavid OpenID account and use it with the same OpenID identity. Thus you could have a Yubikey at home and one at work if you wish so