Be explicit about subdomain_ and maindomain_ urls in plugins

src/pretix/plugins/paypal/maindomain_urls.py

@@ -3,7 +3,7 @@ from django.conf.urls import include, url
 from .views import abort, success
 
 urlpatterns = [
-    url(r'^(?:(?P<organizer>[^/]+)/)?(?P<event>[^/]+)/paypal/', include([
+    url(r'^(?P<organizer>[^/]+)/(?P<event>[^/]+)/paypal/', include([
         url(r'^abort/$', abort, name='abort'),
         url(r'^return/$', success, name='return'),
     ])),

src/pretix/plugins/paypal/subdomain_urls.py

@@ -0,0 +1,10 @@
+from django.conf.urls import include, url
+
+from .views import abort, success
+
+urlpatterns = [
+    url(r'^(?P<event>[^/]+)/paypal/', include([
+        url(r'^abort/$', abort, name='abort'),
+        url(r'^return/$', success, name='return'),
+    ])),
+]