There really isn't much else I can do to test compatibility on my end. We don't use that plugin on ours or any of our client sites. As I mentioned above, please keep in mind that any plugin that changes WordPress core functionality directly related to registration will likely conflict, so unfortunately that takes the resolution of this issue out of my hands.
Plugins can only function reliably when the site is operating according to the WordPress system of hooks, actions and filters as outlined in the WordPress documentation. Plugins bring a lot of freedom to a site owner and can add or remove features, but as a user it's important to realize that this can affect other plugins, and prevent them from working in one or more areas.
It sounds like the other plugin needs to add some kind of registration spam protection. As with all WordPress plugins, it's going to require that users test and see which combinations work for you.
As there is not an error with WP-SpamShield, I'm going to mark this topic as resolved.