Reset the resource file chain before calling PyMac_OpenPrefFile. I'm not sure why this wasn't a problem before...
diff --git a/Mac/Python/gusiconfig.cpp b/Mac/Python/gusiconfig.cpp
index 59b55e0..ec6b57d 100644
--- a/Mac/Python/gusiconfig.cpp
+++ b/Mac/Python/gusiconfig.cpp
@@ -96,6 +96,7 @@
 		resource_id = GUSIOPTIONSOVERRIDE_ID;
 	} else {
 		/* Next try normal resource from preference file */
+		UseResFile(oldrh);
 		prefrh = PyMac_OpenPrefFile();
 		h = Get1Resource('GU\267I', GUSIOPTIONS_ID);
 		if ( h ) {