For everyone’s benefit:
I noticed that in Sebastian’s XIB there was some strange additional data added to the radio buttons having to do with an alternate image defined on them, even though that didn’t appear in the Xcode interface. It seemed to choke on these lines:
<object class="NSCustomResource" key="NSNormalImage" id="148917631">
<string key="NSClassName">NSImage</string>
<string key="NSResourceName">NSRadioButton</string>
</object>
<object class="NSButtonImageSource" key="NSAlternateImage" id="500300646">
<string key="NSImageName">NSRadioButton</string>
</object>
Other .xibs with NSMatrix Radio groups didn’t seem to have the “NSCustomResource” line on them defined.