This appears to be a bug in splunk, as changing setting there seem to "trick" this into working.
If I set up a new slack alert to a #channel - it throws the 500's.
If I set up a slack alert with default (blank) channel, it triggers properly, and ~50% of the time, once a successful message was triggered to default endpoint, the alert can be set to a different #channel and it will trigger correctly.
I have not tried setting this up specifically with full admin permissions, however it seems a fairly large gap to have to grant full admin rights to something that simply need to post messages.
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