Judging by these lines in the stacktrace, it looks like the problem is with loading the checkpoint:
File "/apps/pcehr/splunk/etc/apps/Splunk_TA_nessus/bin/splunk_ta_nessus/splunktaucclib/data_collection/ta_checkpoint_manager.py", line 31, in get_ckpt
return self._store.get_state(key)
File "/apps/pcehr/splunk/etc/apps/Splunk_TA_nessus/bin/splunk_ta_nessus/splunktalib/state_store.py", line 141, in get_state
state = json.load(jsonfile)
Try looking at the steps in here to reset the checkpoint :
https://docs.splunk.com/Documentation/AddOns/released/Nessus/Troubleshoot#Checkpoint_behavior
If you want to reset the checkpoint, change the start_date for your Nessus input or start_time for your Security Center input. The Splunk platform deletes the checkpoint file and re-indexes your data starting from the new start date.
The checkpoints for nessus:scan and nessus:plugin inputs in $SPLUNK_HOME$/var/lib/splunk/modinputs/nessus/
The checkpoints for tenable:sc:vuln in $SPLUNK_HOME$/var/lib/splunk/modinputs/tenable
(the checkpoint for tenable:sc:vuln was actually in $SPLUNK_HOME$/var/lib/splunk/modinputs/tenable_sc on my local environment)
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