Not sure why that would be, but I found that most shcluster issues can be solved as follows;
Make sure the problem system is not currently the captain (restart service?).
On the captain, remove the problem member from the cluster;
/opt/splunk/bin/splunk remove shcluster-member -mgmt_uri https://problemcaptain.sh:8089
now, on the problem system, shut down splunk (service splunk stop or /opt/splunk/bin/splunk stop or whatever)
then clean it's data and caches etc;
/opt/splunk/bin/splunk clean all (note: most of the config cleared is the replicated config that will re-appear once you rejoin the cluster. But the admin password is cleared as well)
Then start splunk and rejoin to the cluster from the captain;
/opt/splunk/bin/splunk add shcluster-member -new_member_uri https://problemcaptain.sh:8089
The captain looks like it is attempting to post something to itself and failing. I don't see this with the other members of the shcluster. This could be password related. Any idea what password is used for this POST?