diff --git /usr/lib/python2.7/dist-packages/univention/management/console/protocol/message.py /usr/lib/python2.7/dist-packages/univention/management/console/protocol/message.py
index 6db48dc388..43c1333fd0 100644
--- /usr/lib/python2.7/dist-packages/univention/management/console/protocol/message.py
+++ /usr/lib/python2.7/dist-packages/univention/management/console/protocol/message.py
@@ -119,7 +119,10 @@ class Message(object):
 		if _type == Message.REQUEST:
 			type = b'REQUEST'
 		if mimetype == MIMETYPE_JSON:
-			data = json.dumps(body)
+			try:
+				data = json.dumps(body)
+			except ValueError as exc:
+				raise Exception(repr(body), str(exc))
 			if not isinstance(data, bytes):  # Python 3
 				data = data.encode('utf-8')
 		else: