Index: webinterface_handler_wsgi.py
===================================================================
--- webinterface_handler_wsgi.py	(revision 220)
+++ webinterface_handler_wsgi.py	(working copy)
@@ -333,8 +333,11 @@
                 req.flush()
         except Exception:
             register_exception(req=req, alert_admin=True)
-            req.headers_out['content-type'] = 'text/html'
-            start_response(req.get_wsgi_status(), req.get_low_level_headers(), sys.exc_info())
+            try:
+                req.headers_out['content-type'] = 'text/html'
+                start_response(req.get_wsgi_status(), req.get_low_level_headers(), sys.exc_info())
+            except:
+                pass #this happens when output already started, but wan't to show something eventually
             return generate_error_page(req)
     finally:
         for (callback, data) in req.get_cleanups():