Lost heartbeat on node

Symptom

From ambari-agent.log

WARNING 2016-05-16 18:31:30,276 base_alert.py:140 - [Alert][ams_metrics_monitor_process] Unable to execute alert. Unable to find 'AMBARI_METRICS/package/alerts/alert_ambari_metrics_monitor.py' as an absolute path or part of /var/lib/ambari-agent/cache/stacks or /var/lib/ambari-agent/cache/host_scripts

ERROR 2016-05-16 18:31:40,013 Controller.py:340 - Unable to reconnect to https://<server>:8441/agent/v1/heartbeat/<server> (attempts=10, details=local variable 'data' referenced before assignment)

ERROR 2016-05-10 20:56:48,446 Controller.py:185 - Unable to connect to: https://<server>:8441/agent/v1/register/dx1090-dat.cluster.local
Traceback (most recent call last):
File "/usr/lib/python2.6/site-packages/ambari_agent/Controller.py", line 130, in registerWithServer
data = json.dumps(self.register.build(self.version))
File "/usr/lib/python2.6/site-packages/ambari_simplejson/__init__.py", line 230, in dumps
return _default_encoder.encode(obj)
File "/usr/lib/python2.6/site-packages/ambari_simplejson/encoder.py", line 202, in encode
chunks = list(chunks)
File "/usr/lib/python2.6/site-packages/ambari_simplejson/encoder.py", line 426, in _iterencode
for chunk in _iterencode_dict(o, _current_indent_level):
File "/usr/lib/python2.6/site-packages/ambari_simplejson/encoder.py", line 400, in _iterencode_dict
for chunk in chunks:
File "/usr/lib/python2.6/site-packages/ambari_simplejson/encoder.py", line 400, in _iterencode_dict
for chunk in chunks:
File "/usr/lib/python2.6/site-packages/ambari_simplejson/encoder.py", line 400, in _iterencode_dict
for chunk in chunks:
File "/usr/lib/python2.6/site-packages/ambari_simplejson/encoder.py", line 323, in _iterencode_list
for chunk in chunks:
File "/usr/lib/python2.6/site-packages/ambari_simplejson/encoder.py", line 382, in _iterencode_dict
yield _encoder(value)
File "/usr/lib/python2.6/site-packages/ambari_simplejson/encoder.py", line 48, in py_encode_basestring_ascii
s = s.decode('utf-8')
File "/usr/lib64/python2.7/encodings/utf_8.py", line 16, in decode
return codecs.utf_8_decode(input, errors, True)
UnicodeDecodeError: 'utf8' codec can't decode byte 0x8c in position 59147: invalid start byte

ERROR 2016-05-10 20:56:48,446 Controller.py:186 - Error:'utf8' codec can't decode byte 0x8c in position 59147: invalid start byte
WARNING 2016-05-10 20:56:48,446 Controller.py:187 - Sleeping for 29 seconds and then trying again