I just had the same error sequence:
2018-06-21 17:24:15,963 ERROR Execution failed: Traceback (most recent call last):
File "C:\Program Files\Splunk\etc\apps\alertAction_runRemoteCommand\bin\modular_alert.py", line 535, in execute
payload = json.loads(in_stream.read())
File "C:\Program Files\Splunk\Python-2.7\Lib\json\__init__.py", line 339, in loads
return _default_decoder.decode(s)
File "C:\Program Files\Splunk\Python-2.7\Lib\json\decoder.py", line 364, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "C:\Program Files\Splunk\Python-2.7\Lib\json\decoder.py", line 382, in raw_decode
raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
when executing an alert action. Reason here was my action had to be configured to output JSON formatted payload, as this is expected by the python script.
So I would guess that one of the python scripts mentioned in your stacktrace expects a JSON formatted file and the data it gets is not in this format
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